My @Quora answer to Where do you prefer to order textbooks from? http://qr.ae/RA7LQp
First I make sure that it's a good textbook and compare it to others: On Choosing Your Own Textbooks [http://boffosocko.com/2011/07/30/on-choosing-your-own-textbooks/]. If it's worth purchasing, I typically use a bulk search/compare engine to find the best price for the book (including shipping). Lately, one of my favorite comparison engines is http://www.gettextbooks.com as it compares and ranks pretty much all of the major online retailers as well as many smaller ones.
However, when making the final purchase, I'll often redirect/reset the affiliate code so that the purchase supports one of several specific charities.