I like
PayQuake personally. I use them myself, and they are very straightforward about rates(Authorize,net is 100% free) and have good service. They also have an option with no set-up or monthly fees (with a higher discount rate to make up for it) that works out very well for businesses that do low volume or are just starting.
I'd recommend
osCommerce for a shopping cart. Changing the interface past the colors requires PHP knowledge, but it's free (and open source) so you could hire someone to work on it for you for the money you'd save on a commercial storefront(or you could just keep the original layout.)