Amazon adjusts its FBA fee structure regularly, and 2026 brought another round of changes to how fulfillment fees are calculated. The exact rate card matters less to your bottom line than a mechanism most sellers never audit: your fulfillment fee is driven by the size tier Amazon assigns your product — and Amazon can reassign it.
Why size tiers are where the money is
FBA fulfillment fees are bracketed by dimensional and weight tiers. Two products that feel similar in your hand can sit in different brackets, and the fee difference per unit is meaningful. When you sell thousands of units a month, a per-unit fee that's $1.40 too high is $14,000 a month you never agreed to pay — and it arrives with no alert, buried in your settlement's fee lines.
The silent reclassification
Amazon periodically re-measures products in its fulfillment centers using automated scanning. Sometimes the new measurement is wrong — packaging fluff, a scan error, a unit measured mid-fold — and your item jumps a size tier. Your fee goes up. Nothing tells you. The first sign is a fatter fee column, which most sellers never open.
The tell-tale signature: the per-unit fulfillment fee for a SKU changes while the product itself didn't. If SKU-A was charged $6.00/unit last week and $6.80/unit this week with no change on your end, that's a size-tier event worth investigating — and often, disputing.
How to check you're not overpaying
- Pull your settlement flat file and isolate the FBA per-unit fulfillment fee lines per SKU.
- Divide each fee by the quantity to get a per-unit rate, and compare the same SKU across time.
- Any SKU whose per-unit rate stepped up without a product change is a candidate for a measurement dispute.
- Cross-check the dimensions Amazon has on record against your product's true packed dimensions.
Getting overcharges back
When a measurement is wrong, Amazon has a process to re-measure and, where warranted, correct the fee. The catch is that you have to notice first, and you have to notice inside the window where it's economical to act. That's a monitoring problem, not a one-time audit — the reclassification can happen any week.
This is precisely the kind of drift Sumhound watches for. Upload your Amazon settlement and it flags any SKU whose per-unit fulfillment fee moved within the period, so a silent size-tier change becomes a line you can see and act on — instead of a slow leak you find months later, if ever.
Note: fee-recovery as a standalone, percentage-of-refund service has largely dried up as Amazon automated more reimbursements and shortened claim windows. The durable value now is catching fee errors early as part of your normal reconciliation — which costs you nothing extra once you're already reading the file.