Egads! Multi Site shopping cart solution

[PandoraH]

I am a total newbie when it comes to shopping carts, and my Google search on multi-site shopping carts gave me more questions then I know what to do with.

Needed:

Multi-Website Shopping Cart for my totally technically challenged client, who has a small online business selling digital products to a VERY small niche market. Her online business is improving however, and a solution other then Clickbank is a current project she has given me.

Host Site Specifics - Currently
My client already has several websites up and running, and needs a low-tech, self hosted solution for multiple web site (all on different domains, on the same host server) shopping cart system to sell eBooks, videos, and audio and recurring memberships.

IE digital content only. The web host is running a flavor of Unix, btw. I mention this only because it seems SOME of the Shopping Cart systems out there require you to host your sites with them, and I'm not looking for a non-unix Script.

Payment Gateway - Currently
My client is currently using, and is happy with Authorize.net. Moving to another gateway provider is not an option she wishes to pursue.

Shopping Cart Solutions - Currently
My client is already using Clickbank for a single ebook product, but wants to stop using them. She also has an account on 1shoppingcart.com, but has never used their services for anything other then email autoresponders (Now meaningless because she uses Aweber for that).

Self-Hosted Scripts Versus 3d Party Shopping Cart platforms.
At this point, it appears that finding a multi-site shopping cart platform that serves as the central database and integrates with Authorize.net isn't going to happen without a recommendation or two.

In addition, *I* am probably going to be the grunt who sets this up, as I am her only acting webmaster at present, and my skills with "web development" are not particular robust when it comes to this kind of task.

If in fact an affordable, multi-site, 3d party shopping cart platform monthly service is more then my client is wishing to pay, I'll need to offer her an alternative scripted service that is hosted on her web host account.

I'm in over my head, and would appreciate any advice you fine folks can provide. I'm not afraid to use Google, but after 3 hours of that today, it seemed a post about this was also in order.