Amazon charges on whichever is greater: your unit’s actual weight or its dimensional weight. Get the dimensions wrong and every unit is overcharged, quietly, for as long as the error stands.
Use the outer dimensions of the item as it ships, at its longest, widest and tallest points.
Dimensional weight is length × width × height, divided by the dimensional divisor — 139 for standard US FBA. Amazon then bills on whichever is greater, the dimensional weight or the actual weight. A light but bulky product is charged as though it were heavier, because it occupies space a heavier item would.
The failure that costs real money is a measurement error in the catalogue. Amazon periodically re-measures units, and if a product sits just over a size-tier boundary it moves up a tier — on every unit, from that point on, until someone notices and files for correction. It does not appear as a fee change on any report you routinely read.
Dimension and fee discrepancies are recoverable, but only inside Amazon’s claim windows. The audit checks your fee preview against your real measurements across the catalogue.
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