UK and EU imports only. US customs value excludes freight (19 CFR 152.102(f)), so for a US entry this calculation is exactly $0.
UK route · worked research
Air freight changes UK duty. It changes US duty by nothing.
Two customs valuations, two different answers to the same reorder question. The panel below works the UK side on eight consignments. The US side is one line long and it is the banner above.
Air freight and the duty you were not quoted
The forwarder quotes the freight. The entry charges the duty.
UK customs value is CIF, so the air freight goes into the base the duty is charged on — but only a statutory percentage of it, fixed by the zone of the departure airport. The rule applies at entry time. The decision is made at reorder time. Nothing sits in both places.
Across 6 booked air shipments the forwarder quoted GBP 50,685.14 of premium over sea. The customs value moved with it, and the entries carried a further GBP 3,206.11 that nobody quoted.
The blended figure is 6.3%, and the blend is the least useful number here. It averages a China porcelain shipment at 35.0% against a zero-rated Indian shipment at 0.0%. Those are not two points on a scale, they are two different worlds, and the seller only ever ships one of them at a time.
Ashcombe porcelain, Shanghai to Heathrow
Two pallets flown to cover a container that will not berth in time. Container DEMU4638122 (Yantian, p90 19 Sep) runs the mug line to zero before it berths. Two pallets flown as a bridge. The forwarder quoted the freight. Nobody quoted the duty.
PVG Shanghai → LHR London · AWB 000-DEMO0114 · Cathay Meridian Air Cargo (fictional carrier) · Halewood Forwarding Ltd (demo broker) · entry 26GBDEMO000000912F
Cargo ready 7 Sept 2026 · flown 9 Sept 2026 → arrived 10 Sept 2026 · sellable at BHX4 Coventry between 14 Sept 2026 and 19 Sept 2026. p50 / p90 from the corridor band in the air-freight reference data. Never a single date.
The headline case, and the one the verified air-freight analysis works through. A 48.1% duty stack meets a 70% zone percentage, and the forwarder's quote understates the bill by a third.
The rule, and the percentage it gives for this departure airport
Departure airport PVG, Shanghai Pudong International Airport — Zone L, so 70% of the air transport cost enters the customs value. APPENDIX 15B
The regulation, the table and the rate
Reg. 111(3), S.I. 2018/1248. “Where goods are transported by air, the cost of the air transport is the percentage of that cost as set out in the document, 'Air Transport Costs to be included in the customs value, version 1, dated 27 November 2018'.” The operational table is CDS DE 5/21 Appendix 15A/15B, republished 24 July 2026 — 1,141 published rows of IATA code to zone to percentage. Its zone percentages are unchanged from the 2018 document the regulation incorporates, which is itself keyed on country of dispatch rather than on airport. The IATA code of the departure airport, as declared in DE 5/21. Never the country of origin of the goods, and never the airport coordinates.
HMRC guidance, updated 15 April 2026: you only apportion the air transport costs, not the cost of the air waybill or other ancillary charges. Sea is not apportioned. Actual freight to the place of introduction, 100%, no table.
Commodity code 6911100090 — Porcelain and bone-china tableware. Combined rate 48.1%: 12% third-country duty plus 36.1% of trade remedy (B999, S.I. 2019/450, geographical area CN). Remedies are scoped to a geographical area and are re-resolved for this origin, never carried across from another. VERIFIED TARIFF
By sea
What the same cargo cost to float
Why this line
Sea is not apportioned. Actual freight to the place of introduction, 100%, no table.
By air
What the forwarder quoted, and what it moved
1,509.6 kg chargeable at USD 5.25/kg
Quoted in full. Not what enters the customs value — the next line is.
Why this line
Fuel and security surcharges are levied on chargeable weight and are transport-related, so they sit inside the apportionable line-haul — not in ancillaries.
70% of £5,927.75 enters the customs value. The other £1,778.32 does not.
The flown sector arrives in the United Kingdom, so reg. 111(3) applies and only the zone percentage of the line-haul enters the customs value.
Air waybill fee, Security screening, Export documentation, Origin terminal handling, Export customs clearance, Pickup and haulage to airport
Why this line
HMRC guidance, updated 15 April 2026: you only apportion the air transport costs, not the cost of the air waybill or other ancillary charges. These enter the customs value at 100%.
Change in the customs value £4,767.45 — charged at 48.1%. That is where the hidden duty comes from, and it is the line no freight quote has.
What flying this consignment actually costs
1 Cash premium
£6,545.77
£10.16 per unit
What the forwarder quotes: the air bill against what the sea leg would have cost, plus the insurance that moves with it.
2 Hidden duty
£2,293.14
£3.56 per unit
What the change in the customs value costs. It appears on the entry, weeks later, under a heading that looks like duty rather than freight.
3 True premium
£8,838.92
£13.73 per unit
£6,545.77 quoted, £2,293.14 not. Across 644 units.
k — the multiplier on the quote
35.0%
Every £1 of air premium actually costs £1.35.
k is bounded above by r, the combined duty rate — 48.1% here. It reaches r x p (33.7%) only where the two modes differ by nothing but apportionable line-haul.
This shipment sits above r x p, which means something in it is 100% dutiable:
- Insurance to the border — £34.19. Insurance is included at cost to the place of introduction. Reg. 111(3) apportions air transport costs; it does not reach insurance.
- Origin ancillaries — £875.37. They enter at 100% and here they are 3.1 times the sea comparator, so they weigh more in the customs value than the apportionment saves.
k is a function of the zone percentage, the duty rate, the ancillary ratio and the sea comparator — and is near-invariant to the rate level. Correcting an 11% freight-rate error in the source analysis moved k from 32.1% to 32.4%. Quote the multiplier with confidence; treat every pound figure as rate-dependent and read its as-of date.
Before any of this — what is in the cartons
All-in air freight on this consignment is GBP 7,573.07 against GBP 3,881.53 of goods — 195% of what is inside the cartons, and 20.8x the all-in sea cost. At GBP 2.57 of FOB value per kilo this is low-value-density cargo, and value density settles the mode question before the tariff is reached.
Both figures are door-to-door on both sides. Quoting a door-to-door air bill against a freight-to-border ocean figure inflates the multiple by about a third and is the usual way this gets misquoted. The sea side here is an FCL rate, which the air-freight reference data puts at the far end of its own 3x-31x range; it calls 8x-15x the honest headline for like-for-like SME volumes, against an LCL comparator.
How this reconciles against the worked example
The worked example in the verified air-freight analysis computes k = 32.4% on the same commodity code, the same origin, the same zone and the same index. This shipment computes 35.0%.
Same: Commodity code 6911100090, origin CN, Zone L at 70%, r = 48.10%, the same USD 5.25/kg index.
Different: The published example assumed origin ancillaries at 9.05% of the line-haul and a sea counterfactual at 16.98% of it. This shipment's own numbers are 14.8% and 4.7% — a fatter ancillary bill against a cheaper FCL sea rate, and both move k the same way.
k is a function of (p, r, a, s), not a constant per origin. The published example's own range for this exact code and origin runs 32.4% (a = 0.0905) to 36.6% (a = 0.4205, the air-freight reference data's evidenced split). This sits inside it. Quote k with confidence and always say which invoice shape it came from.
Per unit, because that is how the decision is made
| MSKU | Units | FOB/unit | Cash/unit | Hidden/unit | True/unit | of FOB |
|---|---|---|---|---|---|---|
| BXM-POR-MUG-350-WHTAshcombe 350ml Porcelain Mug, White — set of 4 | 576 | £4.64 | £7.82 | £2.74 | £10.56 | 228% |
| BXM-POR-DIN-16P-WHTAshcombe 16-Piece Porcelain Dinner Set, White | 68 | £17.80 | £30.02 | £10.52 | £40.54 | 228% |
| Blended | 644 | £10.16 | £3.56 | £13.73 |
Allocated by fob value share. Under a CIF base the customs value depends on the freight allocation, so allocating freight by customs value share is circular. Invisible on a single-SKU shipment, silently wrong on a mixed one. Allocating by weight instead would move the true premium by up to £5.13 a unit on this consignment. Allocating by FOB share makes the premium proportional to value, so every line shows the same premium as a percentage of FOB. Allocating by weight moves it towards the heavy, cheap lines. Both are defensible and neither is circular; customs value share is the one that is. The swing between them is published here so a seller can see how much of the per-unit figure is a choice.
What the carrier actually billed on
Gross weight
1,509.6 kg
governs
Volumetric at 1:6,000
1,217.3 kg
Chargeable
1,509.6 kg
Density
206.7 kg/m³
Chargeable weight is the greater of gross and volumetric weight. Volumetric weight uses the IATA 1:6000 divisor — one cubic metre bills as 166.7 kg — so anything less dense than that bills on the cube. At 7.304 m³ this consignment sits above the 166.67 kg/m³ break-even, so gross weight governs, and the carrier bills what the scale says.
Gross weight governs. The cargo could take this much more cube before the volumetric charge starts to bite — useful headroom when consolidating. Headroom: 1.754 m³.
Every rate on this screen, and where it came from
| Input | Value | As of | Basis |
|---|---|---|---|
| Air line-haulFreightos Air Index, Far East to Europe | USD 5.25/kg | 15 Sept 2026 | INDEX REGIONAL |
| Origin ancillariesForwarder rate card | £875.37 (15% of line-haul) | 20 Sept 2026 | ESTIMATED |
| Sea comparatorThe tenant's own reconciled entry 26GBDEMO000000308M | £278.23 (GBP 0.18431/kg) | 20 Sept 2026 | ENTRY |
| FXHMRC published monthly customs rate, not spot | HMRC monthly customs rate, USD 1.3370 to GBP 1 | 20 Sept 2026 | HMRC MONTHLY |
| Duty ratePorcelain and bone-china tableware | 48.1% = 12% third-country + 36.1% trade remedy | 20 Sept 2026 | VERIFIED TARIFF |
The band, not just the point estimate
Door to door this consignment implies $6.71/kg against a published corridor band of $6.00–$11.00/kg — within the band.
Band as of 31 Aug 2026. Total bill £7,573.07.
What is assumed here, stated as assumption
- This rate card is an assumption, not an invoice. The two published anchors for a disagree by 4.6x (0.0905 in the verified air-freight analysis, 0.4205 in the air-freight reference data's worked example) and a moves k. Replace with the forwarder's itemised invoice at the first opportunity.
- An FCL rate. A consignment this size could not in practice buy FCL on its own. The FCL rate is used because it is the marginal cost of the sailing these units were pulled out of, and because it is evidenced. The LCL alternative is computed alongside rather than described.
Supercargo computes an estimate for management accounting from published tariff data and from documents you supply. It is not customs advice, it is not a classification opinion, and it must not be used as the basis of a customs declaration. You choose the commodity code; Supercargo never proposes one from a product description. Your licensed customs broker remains the decision-maker on classification, valuation and what is declared, and where anything here differs from your broker, your broker governs.
Every air waybill, carrier, entry reference and consignment in this file is invented. Airports, IATA codes, zone percentages, commodity codes, duty rates and freight indices are real, because the demo has to behave like the real thing. Law and rates verified 20 Sept 2026. Every figure on this panel is emitted by a generator that asserts 144 invariants against the underlying dataset — including that the lines in each column above sum to the customs value printed beneath them — and refuses to write the file if any of them fails.
The 144 invariants, in full
- PASS ZONE-AIR-CN-01 — PVG resolves to a zone in Appendix 15B and it matches the shipment
- PASS SUM-AIR-CN-01 — true premium = cash premium + hidden duty
- PASS CV-AIR-AIR-CN-01 — air column: the ADD and STEP rows sum to the customs value
- PASS STEP-AIR-CN-01 — an apportionment step is present only where the zone is known
- PASS CV-SEA-AIR-CN-01 — sea column: the ADD rows sum to the customs value
- PASS APP-AIR-CN-01 — apportioned line-haul + non-apportioned = dutiable freight (air)
- PASS DLT-AIR-CN-01 — hidden duty = the customs-value delta times r
- PASS K-BOUND-AIR-CN-01 — k never exceeds the combined duty rate r
- PASS K-RP-AIR-CN-01 — with r > 0 and p < 1: (k <= r x p) if and only if (a <= s)
- PASS K-SENT-AIR-CN-01 — the '1 pound costs' figure matches 1 + k
- PASS K100-AIR-CN-01 — where k exceeds r x p the panel names what is 100%-dutiable
- PASS STATE-AIR-CN-01 — duty state is one of the four named states
- PASS UNITS-AIR-CN-01 — allocation unit total equals the sum of the shipment lines
- PASS ALLOC-AIR-CN-01 — the per-MSKU premiums sum to the shipment premium
- PASS PU-AIR-CN-01 — blended per-unit true premium x units returns the shipment premium
- PASS RATE-AIR-CN-01 — every rate on the panel carries an as-of date and a basis
- PASS VOL-AIR-CN-01 — chargeable weight is the greater of gross and volumetric
- PASS ZONE-AIR-IN-02 — BOM resolves to a zone in Appendix 15B and it matches the shipment
- PASS SUM-AIR-IN-02 — true premium = cash premium + hidden duty
- PASS CV-AIR-AIR-IN-02 — air column: the ADD and STEP rows sum to the customs value
- PASS STEP-AIR-IN-02 — an apportionment step is present only where the zone is known
- PASS CV-SEA-AIR-IN-02 — sea column: the ADD rows sum to the customs value
- PASS APP-AIR-IN-02 — apportioned line-haul + non-apportioned = dutiable freight (air)
- PASS DLT-AIR-IN-02 — hidden duty = the customs-value delta times r
- PASS K-BOUND-AIR-IN-02 — k never exceeds the combined duty rate r
- PASS K-RP-AIR-IN-02 — r = 0 collapses k to zero regardless of a, s and p
- PASS K100-AIR-IN-02 — where k exceeds r x p the panel names what is 100%-dutiable
- PASS STATE-AIR-IN-02 — duty state is one of the four named states
- PASS UNITS-AIR-IN-02 — allocation unit total equals the sum of the shipment lines
- PASS ALLOC-AIR-IN-02 — the per-MSKU premiums sum to the shipment premium
- PASS PU-AIR-IN-02 — blended per-unit true premium x units returns the shipment premium
- PASS RATE-AIR-IN-02 — every rate on the panel carries an as-of date and a basis
- PASS VOL-AIR-IN-02 — chargeable weight is the greater of gross and volumetric
- PASS ZONE-AIR-IN-03 — DEL resolves to a zone in Appendix 15B and it matches the shipment
- PASS SUM-AIR-IN-03 — true premium = cash premium + hidden duty
- PASS CV-AIR-AIR-IN-03 — air column: the ADD and STEP rows sum to the customs value
- PASS STEP-AIR-IN-03 — an apportionment step is present only where the zone is known
- PASS CV-SEA-AIR-IN-03 — sea column: the ADD rows sum to the customs value
- PASS APP-AIR-IN-03 — apportioned line-haul + non-apportioned = dutiable freight (air)
- PASS DLT-AIR-IN-03 — hidden duty = the customs-value delta times r
- PASS K-BOUND-AIR-IN-03 — k never exceeds the combined duty rate r
- PASS K-RP-AIR-IN-03 — with r > 0 and p < 1: (k <= r x p) if and only if (a <= s)
- PASS K-SENT-AIR-IN-03 — the '1 pound costs' figure matches 1 + k
- PASS K100-AIR-IN-03 — where k exceeds r x p the panel names what is 100%-dutiable
- PASS STATE-AIR-IN-03 — duty state is one of the four named states
- PASS UNITS-AIR-IN-03 — allocation unit total equals the sum of the shipment lines
- PASS ALLOC-AIR-IN-03 — the per-MSKU premiums sum to the shipment premium
- PASS PU-AIR-IN-03 — blended per-unit true premium x units returns the shipment premium
- PASS RATE-AIR-IN-03 — every rate on the panel carries an as-of date and a basis
- PASS PREF-AIR-IN-03 — the preference branch with a valid claim is exactly duty-neutral
- PASS PREF-R-AIR-IN-03 — the shipment's headline rate is the no-claim branch
- PASS VOL-AIR-IN-03 — chargeable weight is the greater of gross and volumetric
- PASS ZONE-AIR-CN-04 — CAN resolves to a zone in Appendix 15B and it matches the shipment
- PASS SUM-AIR-CN-04 — true premium = cash premium + hidden duty
- PASS CV-AIR-AIR-CN-04 — air column: the ADD and STEP rows sum to the customs value
- PASS STEP-AIR-CN-04 — an apportionment step is present only where the zone is known
- PASS CV-SEA-AIR-CN-04 — sea column: the ADD rows sum to the customs value
- PASS APP-AIR-CN-04 — apportioned line-haul + non-apportioned = dutiable freight (air)
- PASS DLT-AIR-CN-04 — hidden duty = the customs-value delta times r
- PASS K-BOUND-AIR-CN-04 — k never exceeds the combined duty rate r
- PASS K-RP-AIR-CN-04 — with r > 0 and p < 1: (k <= r x p) if and only if (a <= s)
- PASS K-SENT-AIR-CN-04 — the '1 pound costs' figure matches 1 + k
- PASS K100-AIR-CN-04 — where k exceeds r x p the panel names what is 100%-dutiable
- PASS STATE-AIR-CN-04 — duty state is one of the four named states
- PASS UNITS-AIR-CN-04 — allocation unit total equals the sum of the shipment lines
- PASS ALLOC-AIR-CN-04 — the per-MSKU premiums sum to the shipment premium
- PASS PU-AIR-CN-04 — blended per-unit true premium x units returns the shipment premium
- PASS RATE-AIR-CN-04 — every rate on the panel carries an as-of date and a basis
- PASS VOL-AIR-CN-04 — chargeable weight is the greater of gross and volumetric
- PASS ZONE-AIR-IN-05 — DXB resolves to a zone in Appendix 15B and it matches the shipment
- PASS SUM-AIR-IN-05 — true premium = cash premium + hidden duty
- PASS CV-AIR-AIR-IN-05 — air column: the ADD and STEP rows sum to the customs value
- PASS STEP-AIR-IN-05 — an apportionment step is present only where the zone is known
- PASS CV-SEA-AIR-IN-05 — sea column: the ADD rows sum to the customs value
- PASS APP-AIR-IN-05 — apportioned line-haul + non-apportioned = dutiable freight (air)
- PASS DLT-AIR-IN-05 — hidden duty = the customs-value delta times r
- PASS K-BOUND-AIR-IN-05 — k never exceeds the combined duty rate r
- PASS K-RP-AIR-IN-05 — with r > 0 and p < 1: (k <= r x p) if and only if (a <= s)
- PASS K-SENT-AIR-IN-05 — the '1 pound costs' figure matches 1 + k
- PASS K100-AIR-IN-05 — where k exceeds r x p the panel names what is 100%-dutiable
- PASS STATE-AIR-IN-05 — duty state is one of the four named states
- PASS UNITS-AIR-IN-05 — allocation unit total equals the sum of the shipment lines
- PASS ALLOC-AIR-IN-05 — the per-MSKU premiums sum to the shipment premium
- PASS PU-AIR-IN-05 — blended per-unit true premium x units returns the shipment premium
- PASS RATE-AIR-IN-05 — every rate on the panel carries an as-of date and a basis
- PASS VOL-AIR-IN-05 — chargeable weight is the greater of gross and volumetric
- PASS ZONE-AIR-CN-06 — PVG resolves to a zone in Appendix 15B and it matches the shipment
- PASS SUM-AIR-CN-06 — true premium = cash premium + hidden duty
- PASS CV-AIR-AIR-CN-06 — air column: the ADD and STEP rows sum to the customs value
- PASS STEP-AIR-CN-06 — an apportionment step is present only where the zone is known
- PASS CV-SEA-AIR-CN-06 — sea column: the ADD rows sum to the customs value
- PASS APP-AIR-CN-06 — apportioned line-haul + non-apportioned = dutiable freight (air)
- PASS DLT-AIR-CN-06 — hidden duty = the customs-value delta times r
- PASS K-BOUND-AIR-CN-06 — k never exceeds the combined duty rate r
- PASS K-RP-AIR-CN-06 — with no apportionment engaged (p = 1), k equals r exactly
- PASS K-SENT-AIR-CN-06 — the '1 pound costs' figure matches 1 + k
- PASS K100-AIR-CN-06 — where k exceeds r x p the panel names what is 100%-dutiable
- PASS STATE-AIR-CN-06 — duty state is one of the four named states
- PASS UNITS-AIR-CN-06 — allocation unit total equals the sum of the shipment lines
- PASS ALLOC-AIR-CN-06 — the per-MSKU premiums sum to the shipment premium
- PASS PU-AIR-CN-06 — blended per-unit true premium x units returns the shipment premium
- PASS RATE-AIR-CN-06 — every rate on the panel carries an as-of date and a basis
- PASS ALT-AIR-CN-06 — the cheapest flown leg is not the cheapest entry
- PASS VOL-AIR-CN-06 — chargeable weight is the greater of gross and volumetric
- PASS ZONE-AIR-US-07 — HKG resolves to a zone in Appendix 15B and it matches the shipment
- PASS SUM-AIR-US-07 — true premium = cash premium + net government charges
- PASS CV-US-AIR-US-07 — both US columns reach an identical customs value
- PASS DLT-AIR-US-07 — US duty delta is exactly zero
- PASS STATE-AIR-US-07 — duty state is one of the four named states
- PASS UNITS-AIR-US-07 — allocation unit total equals the sum of the shipment lines
- PASS ALLOC-AIR-US-07 — the per-MSKU premiums sum to the shipment premium
- PASS PU-AIR-US-07 — blended per-unit true premium x units returns the shipment premium
- PASS RATE-AIR-US-07 — every rate on the panel carries an as-of date and a basis
- PASS MPF-AIR-US-07 — the MPF minimum binds on every air entry and the ad valorem on the sea entry
- PASS MPF-SUM-AIR-US-07 — the air MPF total is the sum of the per-entry charges
- PASS HMF-AIR-US-07 — air escapes the harbor maintenance fee entirely
- PASS GOV-AIR-US-07 — net government charges = MPF extra + brokerage extra - HMF saved
- PASS UNK-AIR-US-07 — the unclassified line is carried as unknown, and still answers the mode question
- PASS VOL-AIR-US-07 — chargeable weight is the greater of gross and volumetric
- PASS ZONE-AIR-CN-08 — HKG resolves to a zone in Appendix 15B and it matches the shipment
- PASS SUM-AIR-CN-08 — true premium = cash premium + hidden duty
- PASS CV-AIR-AIR-CN-08 — air column: the ADD and STEP rows sum to the customs value
- PASS STEP-AIR-CN-08 — an apportionment step is present only where the zone is known
- PASS CV-SEA-AIR-CN-08 — sea column: the ADD rows sum to the customs value
- PASS APP-AIR-CN-08 — apportioned line-haul + non-apportioned = dutiable freight (air)
- PASS DLT-AIR-CN-08 — hidden duty = the customs-value delta times r
- PASS K-BOUND-AIR-CN-08 — k never exceeds the combined duty rate r
- PASS K-RP-AIR-CN-08 — with r > 0 and p < 1: (k <= r x p) if and only if (a <= s)
- PASS K-SENT-AIR-CN-08 — the '1 pound costs' figure matches 1 + k
- PASS K100-AIR-CN-08 — where k exceeds r x p the panel names what is 100%-dutiable
- PASS STATE-AIR-CN-08 — duty state is one of the four named states
- PASS UNITS-AIR-CN-08 — allocation unit total equals the sum of the shipment lines
- PASS ALLOC-AIR-CN-08 — the per-MSKU premiums sum to the shipment premium
- PASS PU-AIR-CN-08 — blended per-unit true premium x units returns the shipment premium
- PASS RATE-AIR-CN-08 — every rate on the panel carries an as-of date and a basis
- PASS VOL-AIR-CN-08 — chargeable weight is the greater of gross and volumetric
- PASS LBL-1 — every shipment in air-demo.json has a decision label, and no label is orphaned
- PASS K-RANGE — the panel's k range matches the dataset summary
- PASS STATE-COVER — the panel covers all four duty states the brief names
- PASS CASE-COVER — the panel covers every case the brief says must not render as a bug
- PASS ZONE-UNKNOWN — no departure airport in this dataset is missing from Appendix 15B, so the unknown-zone branch is unreachable here and is built anyway
- PASS FICTION — every waybill, entry and carrier stays inside the demo naming conventions
- PASS DECLINED — a quote that was turned down is still carried as a decision
- PASS SCRUB-1 — no prose the panel renders names an internal file or path
Why this sits on a UK route
The same shipment, two customs values
Why the UK and US answers differ
UK customs value under the transaction-value method is CIF: the price of the goods plus international freight and insurance to the border. So the freight is inside the base the duty is charged on, and changing the mode changes the duty. The United States excludes transport, insurance and related services from the price actually paid or payable under 19 CFR 152.102(f) — and excludes them from every layer built on top of it, from the MFN rate through Section 301, Section 232 derivative duties and AD/CVD. A sea-to-air switch on a US entry moves the duty by exactly zero.
That is not a rounding difference, it is a sign change, and it is why this panel has its own route rather than a tab in the demo. Supercargo is US-first. A screen that computes duty on freight has no business appearing unlabelled in front of an importer whose customs value does not contain any.
The two asymmetries that survive in the US
Two asymmetries do survive in the US, and both are small enough that they belong in a footnote rather than a headline. The harbor maintenance fee under 19 CFR 24.24 is 0.125% of value on cargo moving by commercial vessel, so air escapes it. And the merchandise processing fee under 19 CFR 24.23 has a per-entry floor, so three smaller air consignments carry three floors and three broker fees where one ocean consignment carried one. Neither is a duty effect.
For a single code on either side of the Atlantic, the landed cost calculator works on published rates.
Start with the audit
Your last twelve months of entries, reconciled to the unit, for a fixed £900$1,200€1,050. Credited in full against your first year, and refunded in full if the variance is under 2%.