When I signed for prime 3-4 years ago there was also a lot online retailers jerking around on shipping prices. "Oh that CD is $0.01 but it is $11.99 shipping, so if you don't like it then you'll be refunded the purchase price." Shipping prices in general seemed to be where quite a bit of profit was being made.

I don't know if that is still the case but it was a major reason for me to move to prime back then.