I must bought over $100 - $150 worth in books from free amazon credit that I somehow got! It was as if I was a child in a book..... store. lol ;0
Amazon sent me an email that said that I had a certain amount of credit and that they would take that much off my next purchase. So I went and bought a book and they didn't take the credit off. Then I got another email saying I had more credit towards my next purchase.
I think the credits are only good for certain items, but I don't know how to tell which items qualify.