Most FBA calculators give you a single number. When it's wrong, by a stale rate card, a missed surcharge or a box billed on dimensional weight, you find out from your settlement report, after the inventory is bought.
Feeproof shows the work instead: the tier, the weight Amazon bills on, the rate-card row, the price band, every surcharge, and a label on each line saying whether it is Amazon's published rate, something we derived, or your own input. The fee data lives in one file that both the calculator and the published tables read, and it is covered by automated tests built from Amazon's own examples.
We are not affiliated with Amazon. The calculators are free, need no account and don't send your numbers anywhere: everything runs in your browser.
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Read the methodology or the changelog.