Analysis · Verified against Amazon Seller Central, 27 August 2026
Most margin calculations we see leave out four real costs. The gap is usually the difference between a campaign that makes money and one that quietly doesn’t.
What this is. A worked model on an invented product, using Amazon’s published 2026 rate cards. The product isn’t real. Every fee figure is, and you can check the arithmetic against your own Seller Central account. We publish it because it’s the calculation that decides whether your advertising is profitable, and the version in circulation is wrong.
The product
A kitchen storage item. £34.00 ex-VAT in the UK, shipped FBA, Large standard size tier, 1.4 kg packed.
The calculation most people run
| Line | Amount |
|---|---|
| Selling price | £34.00 |
| Landed COGS | −£9.60 |
| Referral fee, Home & Kitchen at 15% | −£5.10 |
| FBA fulfilment fee | −£3.05 |
| Gross profit | £16.25 |
| Break-even ACoS | 47.8% |
Four lines. It’s the version in most FBA course material, and it overstates your break-even by about two and a half points.
The calculation that survives contact with a P&L
| Line | Amount | Where it comes from |
|---|---|---|
| Selling price | £34.00 | |
| Landed COGS | −£9.60 | Manufacture, freight and duty to warehouse |
| Referral fee, 15% | −£5.10 | Amazon UK fee schedule |
| FBA fulfilment fee | −£3.05 | 2026 UK rate card |
| Monthly storage, amortised | −£0.18 | £0.75/cu ft Jan–Sep; roughly triple in Q4 |
| Returns processing, amortised | −£0.41 | Charged above the Home & Kitchen threshold of 8.1% |
| Refund administration | −£0.12 | Lesser of £5 or 20% of the referral fee, per refund |
| Digital services fee, 2% | −£0.17 | Applied to Selling on Amazon fees for UK sellers |
| Gross profit | £15.37 | |
| Break-even ACoS | 45.2% |
The gap is 2.6 percentage points. On £200,000 of ad-driven revenue that’s roughly £5,200 of margin the four-line version says you have and you don’t.
Why those four lines get missed
They’re small individually and none of them appears on the order. Storage is billed monthly against total volume, not per unit. Returns processing is charged between the 7th and 15th of the third month after the sale, by which point nobody connects it to anything. Refund administration shows up as its own line in the transaction report. And the digital services fee is a percentage uplift most UK sellers have never gone looking for.
Each is under 0.5% of the sale price. Together they’re 2.6% of the margin.
The seasonal trap
Storage is the line that moves. UK and US standard-size storage roughly triples between October and December, and the fee is set when the unit ships rather than when it was ordered.
A break-even ACoS worked out in July and still in use in November is describing a product that no longer exists. Model the Q4 version separately, or you’ll run Black Friday against a summer margin.
What this means for the ad account
At 45.2% break-even, a campaign running at 30% ACoS retains about fifteen points of margin per order. A campaign at 47% loses money on every unit while looking perfectly healthy on a dashboard that has no idea what your COGS is.
Amazon states the principle plainly: “Break-even ACOS is directly linked to your profit margin. In order to maintain a profit, your Amazon ACOS needs to be lower than your profit margin.” What Amazon can’t tell you is what your margin actually is. That part you build.
Check your own
Amazon publishes your real numbers. The SKU Economics report gives actual charged fees at MSKU level. The Fee and Economics Preview report estimates up to 120 days forward, including announced fee changes if your date range covers the effective date.
Run one SKU through both and compare against whatever figure you’ve been using. If the gap is more than a point, every campaign target built on the old number is wrong in the same direction.
Not sure your break-even is right?
Our forensic audit rebuilds unit economics per SKU from your own fee reports, then checks every campaign target against it. Most accounts find at least one product being advertised at a loss.
Prepared by Ecom Enable. Fee figures verified against Amazon Seller Central UK on 27 August 2026. This is an illustrative model using published rates on an invented product — not a client account, and not a projection of results. Amazon changes its fee schedule annually; check the live rates before acting on any figure here.