I've had to return a book once. This was 2 Christmases ago, but the procedure at that time was that you print off a shipping label from their website, which you can put on the original box. They'll refund your money for the product. We were still out the shipping cost, but I think that was because it was our error not Amazon's. In your case, hopefully you don't have to pay shipping.
ETA: nevermind, poster above beat me to it!