But amazon didn't take the bullet. The seller of the mattress took the bullet.
Amazon didn't lose a penny.
Sellers just have to assume a 10% loss of gross margin due to issues like this. It's not the end of the world, but that's where the cost is.
This explains why individuals selling individual items, or a small number of items, can get so burned too. You need sufficient volume to cover all the inevitable bullets you have to swallow.
Cost of doing business. Reminds me of the book "What I Learned Losing a Million Dollars" where Jim Paul describes how someone trading in lumber doesn't cry about 20% of the wood being useless, he knows 20% will be useless and he writes that up under cost of doing business. Just like refunds and customer support are a cost of doing business.