My @Quora answer to Where do you prefer to order textbooks from?
First I make sure that it’s a good textbook and compare it to others: 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 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.