FYI Giftcertificates.com is partnered with many many rewards programs (airline and non-airline) - see http://rewardsdb.com/product/Gift-Certificates.html for a list. So this approach (borders gift cards from GiftCertificates.com redeemed at Amazon.com) may work for people interested in a broader range of miles/points etc.

Of course as has been mentioned, you do need to look at the fees and balance these with buying the product from someone like Barnes & Noble who are directly linked to many programs (see http://rewardsdb.com/product/Barnes-and-Noble.html)