Can anyone share insight on the current good deals in "Turn Key" shopping carts?

I have several software/document deliverables that can be fulfilled online, so there is no need for shipping of a physical product.

The shopping cart needs to have a store front linked from my site, accept credit cards, and then deliver the product online after the payment has cleared.

The market won't be huge, so I'm really looking past storefronts / credit card clearing houses that charge an arm and a leg.

So what are the good deals and/or best way to handle this?

Thanks!