Amazon provides free 2-day shipping for a year if you have a student email, one free month trial if you don't, and they will pay return shipping regardless of size or weight. They don't give you any hassles, if you want to return something within 30 days, they will take it back, and I've seen exceptions made for extensions.
There aren't many retailers like Amazon online. Newegg, for example, is a typical online retailer. They don't pay for return shipping, they've got about 50 return policies on their site, most of them very stringent, and many of them violates some sort of credit card regulation (i.e., they bank on the fact that most buyers are clueless about their rights.).