Not a broker and not a lawyer, and I can’t see inside Shopify’s calculator either — but several things changed on both ends since this thread started, and any comparison made before them won’t reconcile.
1. The PDDP ceiling moved. Since 24 July 2026 the maximum eligible value for a PDDP (prepaid duty) shipment to the USA went from $800 to $2,500 — it’s in the timeline of changes on Royal Mail’s USA business updates page. As of 5 August 2026 their own PDDP product page still says, in the “What restrictions apply?” section, that the service to America is “only available for packages up to $800”. So whichever number you’ve been designing around, one of Royal Mail’s two pages is stale; worth saving a screenshot of the updates page before any support conversation.
2. What’s charged on the postal lane changed the same day. Royal Mail’s timeline of changes says that from 24 July “standard duty rates, including any applicable Section 232 and 301 duties such as Forced Labour, will apply” (the FAQ further down that same page drops the “such as Forced Labour” wording). On the US side the 10% Section 122 surcharge wasn’t cancelled — it ran out: the proclamation set it to run “through 12:01 a.m. eastern daylight time on July 24, 2026” (91 FR 9339), and the tariff schedule now notes those headings expired at the close of 23 July. An additive 10% under the new Section 301 action took effect the same minute (91 FR 47318); UK goods sit in the additive-10% band, heading 9903.05.81.
Same headline 10%, completely different legal basis — but the two exemption lists are not the same, so “nothing changed” only holds if your goods were exempt under both or under neither. Section 301 has a UK-specific exclusion (heading 9903.05.96 — Scotch whisky and certain medical devices, which did pay the old surcharge), a general exclusion list that doesn’t match the old one, and an informational-materials exclusion (9903.05.92) covering publications, posters, photographs and artworks — those pay nothing under either regime, so for prints the answer is 0%, not 10%. Two more things that move the number: it keys on country of origin, not where you post from (stock you resell that was made elsewhere sits in a different band, which is a very common reason a checkout number and a carrier number disagree), and goods already in transit on 24 July and entered before 28 July escaped both.
3. The US end of the mail channel is a different process now. Mail shipments valued at $2,500 or less can use a new informal entry process that took effect 24 July 2026 (91 FR 37801). $2,500 isn’t a new allowance, by the way — it’s the long-standing informal-entry ceiling; what changed is that the $800 de minimis exemption is gone for mail. For parcels going through that process duties are no longer collected from the recipient on delivery: they’re reported on a monthly worksheet with 10-digit HTS codes and paid through Pay.gov by the 7th day of the month following the parcel’s arrival. The filer has to be the owner or purchaser, or a licensed customs broker they designate, and must hold a basic importation and entry bond — no bond, no release (19 CFR 145.15). Alcohol and tobacco need formal entry regardless.
Watch the shelf life on that one: the same rule makes anything carrying a chapter 98 or 99 duty ineligible for the postal informal process, and CBP delayed the compliance date for that requirement to 22 October 2026 (19 CFR 145.12(a)(2)(vi)). UK-origin goods now carry a chapter 99 heading — the one above — so unless CBP moves that date, UK parcels look set to fall out of the postal informal process in October. Worth knowing before you build a process around it. Royal Mail’s own updates page also says the next phase of US changes is due in September 2026, so this isn’t settled yet.
On fees, since they’re easy to mix up: the 50p per-item figure everyone quotes is published under the heading “Account PDDP Handling Fees” on Royal Mail’s PDDP page — it’s the account tariff. For sending without a credit account Royal Mail publishes no figure at all today; the consumer page just says the service is available online and in all UK Post Offices and that “a small service fee will apply”. The only number I’ve found for the non-account route is £1.50 per package, in a September 2025 Royal Mail PDDP Q&A — but that document still describes the old $800 cap, so I wouldn’t treat it as current, and sellers have reported the £1.50 at Post Office counters rather than online. The fee is shown before you pay, so check what your own route actually charges rather than assuming 50p.
What I’d actually do rather than reverse-engineer the difference: ask Royal Mail in writing which value they compute duty on and which chapter 99 headings they’re applying to US-bound parcels, and keep the reply. Carrier practice has been inconsistent enough this summer that a written answer is worth more than a calculator comparison — and it’s the only thing that helps if you later have to dispute a recharge invoice.