I'm looking to e-commerce enable a couple of sites I'm working on and I need to know what steps I should go through to get things going.

Basically here's what I'm looking for:

Two shopping carts (for two different sites) that allow around 20 items each and are very customizable.

The shopping carts should allow clients to download the product online after they have purchased it and their credit card has been verified.

Now...what do I really need in order to add these to my sites?

I wouldn't mind if there were a 3rd party transaction company involved (paypal, clickbank, etc.) but as far as I know, none of them offer these types of services (especially the download-after-payment service). If there's a script that will do it for me, that would be fine too...and then I'd need to hire a programmer

Please help me get started