Are you looking for something quick and simple, or something robust and complex (i.e. are you doing something for a big existing business, or something new)?
There's a ton of hosted shopping carts out there, unfortunately I haven't used any. They're usually the easiest way to get started.
For a bigger project for an existing business there's software that you would install and operate on your own server.
For hosted ones that you just sign up for:
http://webstore.amazon.com/
http://smallbusiness.yahoo.com/ecommerce/
http://www.storefront.net/
http://www.shopify.com/
I haven't seen much in any of that about a user rewards system however, that might be something harder to find.
Some software out there does this:
http://www.prestashop.com/en/
But you have to setup the software yourself and do the design, so you'd still need someone to do that.
They have a forum where people can setup that kind of stuff:
http://www.prestashop.com/forums/vie..._paid_services