Shopping Cart using external data

I'm wondering if anyone has managed to set up a shopping cart in Drupal that utilizes an external product database. I'm trying to set up an online cart that sells products from a vendor that uses XML based Web Services to feed data to our site.

By this, I mean that I can take user input and look up a product in an external database (via an HTTP request) and receive a response from our vendor via XML. I would like to feed that XML response to our shopping cart. I don't want to set up a copy of all that product data in MySQL.

I can handle the programming involved in parsing the XML from our vendor. I just can't find a shopping cart that will take product info from anything but a local database.

I've been looking through Drupal's forums and search results, as well as Google's and I've come up empty. I hope it's because my search terms are off. Surely somebody, somewhere has had to set up a store where they use a vendor's data for their shopping cart.

I'd be happy to use any of the popular carts for this purpose, but all my searching tells me that I need to cram the data into a locally hosted database. That doesn't make any sense, since the vendor is willing to provide that data (3million products or so...) without my having to copy it locally.

I don't want to just import the Vendor's data into a database. They have a gazillion items and updates occur daily. I want to make 'realtime' requests against THEIR data -- which they allow via http/SOAP requests.

Aren't there any carts out there that will allow for the data to be fed from an outside source? Help!

Thanks.

-dave.