Backend Website Automation

I am looking for backend website automation that will allow the following. Use a shopping cart feature to allow customers to order free/lowcost items from my website. They would click on each item they want and it would be added to their shopping cart. I want to be able to limit the items they can get The instructions needed to get the free items will be delivered via email automatically to the customer. eg. Free pencil is chosen, once customer checks out they will receive an email from me stating exactly what they must do to get the pencil. If they buy a low cost item they would get confirmation that item will be shipping soon Must be able to use all common payment gateways (eg paypal etc.) If item they buy is an item that can be electronically delivered I want that done automatically I need to be able to charge vendors to list their free item I need to be able to send customer names to vendors I need to be able to use this shopping cart system for several of my websites and need the ability to change the colors and graphic to match my website.