I'd recommend VP-ASP. I've been using it for two years and, having tried OS Commerce and a few others, really like it.
It's highly customizable, supports just about every payment gateway available and the back end pages are heads above most competitors. It's portable and you can do what ever you want with it. Support is pretty good and there are tons of 3rd party add-ons as well.
Try out the free demo, it's got limited functionality but it'll allow you to get a feel for it. The paid versions are very reasonably priced and full of some pretty amazing features.
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