A fee that crept up and nobody saw it
Your best-selling phone case has always been in a small standard size tier. At some point Amazon remeasured it, or measured a unit in a slightly different box, and recorded it one tier up. The fulfilment fee on every order went up. Your margin on that SKU fell. Nobody noticed because the fee is deducted before the money reaches you, buried in the settlement among many other fee lines.
Months later your accountant asks why gross margin has slipped on your core range. Finding the answer takes an afternoon of digging through reports, and fixing it takes a support case with measurements and photos.
Why fee errors are easy to miss
FBA fees are calculated per unit from Amazon's own record of each product's dimensions and weight. That record can change when Amazon remeasures, when packaging varies between batches, or when a product is received differently than expected. Fee rate changes, which Amazon announces, get mixed up with measurement changes, which it may not highlight.
- Fees are deducted automatically, so there is no invoice to check.
- Settlement reports show fees in total, not per SKU against expectation.
- Rate changes and measurement changes both raise fees and look the same.
- Your own product dimensions sit in supplier specs, not next to Amazon's figures.
- A small increase per unit is invisible until you multiply it by a year of sales.
The cost of letting it run
An overcharge on a high-volume SKU runs on every single order until someone spots it. It also distorts decisions: a product can look marginal when the real problem is a wrong fee, and you might cut a line that was profitable. Queries raised long after the change are harder to support, because the packaging may have changed and your evidence is thin.
Then there is the planning cost. If your pricing and profit model assumes one fee and Amazon charges another, every forecast built on it is slightly wrong.
The fee check we build
- A product table holding your own measured dimensions and weight for each SKU as packaged for FBA, with a photo of the measurement where you have one.
- Regular reads through the Selling Partner API of Amazon's recorded dimensions, weights and fee previews for each SKU, plus the fees actually charged per order from your financial events.
- The size tier your figures suggest, calculated from Amazon's published size tier rules, which are held as settings so they can be updated when Amazon changes them.
- A comparison that flags any SKU where Amazon's recorded tier, dimensions or charged fee differ from what your figures suggest.
- A separate note when fees rose because of an announced rate change rather than a measurement change, so the two are not confused.
- A prepared query for each flagged SKU with both sets of measurements, the dates the fee changed and example orders, for a person to check and raise with Seller Support.
| Finding | What it usually means |
|---|---|
| Amazon dimensions larger than yours | Remeasured or different packaging batch |
| Fee rose on all SKUs at once | Announced rate change |
| Fee rose on one SKU only | Likely measurement or tier change |
| Tier matches but fee differs | Check category or fee type |
| Your figures missing | Measure before drawing conclusions |
The check does not decide Amazon is wrong. Sometimes your packaging really has grown. The point is to see the difference early and decide with the facts in front of you.
After the check is running
Each month you get a short list of SKUs whose fees have moved for reasons other than an announced rate change. Your team measures a unit, compares with Amazon's record and raises a query where the difference is real. SKUs that genuinely changed packaging are updated in your own table, which keeps the margin model honest.
Your product table also becomes a reliable place for dimensions and weights, which helps with shipment planning, courier quotes for FBM orders and conversations with suppliers about packaging size.
Signs your FBA fees need checking
- Margins on core SKUs have slipped and nobody can say why.
- You do not hold your own measured dimensions for every SKU.
- Fee changes are only noticed at year end.
- Packaging changed between supplier batches without anyone updating records.
- You have never compared Amazon's recorded size tier with your own figures.