I like the simplicity of Coffee Cup's offering but I'm afraid it's still six months to one year away from being a mature shopping cart system.
My sentiments exactly.
Like yourself, I have tried a multitude of shopping carts out there, but find that most of them offer no direct route from the checkout button to the payment processor. I currently use Zen Cart, for which I've had to hand code some of the process templates to get it to work more effectively.
If the pro version of Shopping Cart Creator finally offers a weight based shipping system, you could simply have a weight band of up to one ounce, assign a zero postage cost to that weight band and put a weight of less than one ounce against your digital goods. One of the items I sell, has a specialist polystyrene mailer, so I just put that item in the next weight band up to cover my extra costs. Until this cart has weight based shipping system in place, it loses a massive amount of flexibility and function.