Fulfilment and fees
What is Size tier?
Size tier is the category Amazon assigns a product based on its packaged dimensions and weight, and it determines the FBA fulfilment fee. Dimensions are checked before weight.
Why it matters
Amazon sorts the three dimensions longest to shortest and compares them against the tier table. Small standard requires the longest side at 15 inches or under, the median 12 or under, the shortest 0.75 or under, and at most 16 ounces. Exceeding any one of those drops you to large standard.
This is why packaging is a margin lever. Trimming the shortest dimension to move a unit from large standard into small standard is often worth more than a supplier negotiation, and costs nothing per unit thereafter.