cart scripting with some design

Starting from scratch...we need a cart script either built from scratch or use an existing cart software such as osCommerce or Zen Cart. We are open to suggestions. Please don't bid unless you can get this to do what we need it to do. Here's a scenario of what will happen: A visitor comes to the site, before they can shop, they must either enter membership number (provided by us) or use an existing organization's membership number (of which is listed in an info block on either side) which will get them into the catalog. Once inside, this number will be used on any purchases made by that person to track commissions to that number. If they do not want to support any one of the organizations and do not have a membership number, then they can fill out a form requesting their own membership. Organization's also need the ability to log in (along with anyone with a membership number) and see their commission's coming to them. The membership number doesn't need to have a password associated with it but it just has to be able to be tracked with whoever used it right up to checkout. We want to be able to use Paypal and [login to view URL] for credit card processing. We would like the shipping figured into the final price obviously and would like the option of using either UPS or USPS. The catalog must be able to have thumbnailed images, description, pricing, and when a customer clicks the thumbnail, it will display a larger more in depth image. We may need some help with a simple design of a logo and some graphics, nothing fancy or too in depth but somewhat professional. We are open to any suggestions and are starting from scratch. We don't have an existing site to fit this into. We can install it if needed but would be willing to pay for that also. We expect that whoever we award the project to, that they will help with any bugs for at least 60 days. Thanks