[cookiecaper]

Sellers can opt-out of commingling, but many don't because a) they don't understand the implications, as Amazon doesn't really make it clear; and b) because it requires them to individually add a separate sticker/identifier to each unit of their product, adding significantly to preparation and packaging costs.

[AdamJacobMuller]

I think Amazon charges more for storage fees if you disable commingling too.

[cnst]

I'd imagine fulfillment would be pricier, too, since items could no longer be located at every warehouse at no extra effort.

[throwawaylalala]

Not correct. They do however make you pay for a tier above their "small and light" for handling.