If you want to pay a company a monthly fee for the cart, go with Volusion. It's about 40 to 100 bucks a month. If you want to purchase a cart outright so you can make your own customizations, go with ASPdotnetstorefront. It costs about a grand.
Alot of people here will say use the free carts, which in my opinion is just fine for some smaller websites. However, if you need a cart with a professional feature set that can handle any type of volume, don't expect it to be free.
And yes, everyone here will tell you that you can get "mods" and that you can customize oscommerce and zen cart. Unless you can code this stuff yourself (And I dont mean using HTML) these free carts simply cant compare to the professionally marketed carts that I listed above.
Do the research yourself, but this is solid advise from someone in eCommerce for a little over four years. I've tried most of the carts out there and there simply is no comparison.
You do, or you do not. There is no try.