They've kept it up for several decades and in cases where it's not shipping damage or error (wrong item sent by amazon) they mostly pass the cost off to the sellers afaik. They also have their second hand stores where they sell returns and can recoup a lot of that loss. The ease of returns is one of the main things that keep me from completely cutting off amazon purchases and they probably know that and accept the losses for the gains.

Every time I've returned something to Amazon it was because of their fuck up (sadly a lot more common lately). They send me the completely wrong item, the item arrives broken, etc.