Manage Amazon cart?

This module does a great job of providing search facilities, and it's easy to see looking at the code how to extend the searching facilities.

My question: I'd like to not let the customer escape from my site until they want to check out, so not send them to the Amazon site until checkout time. Has there been any work done in this area? I know lots of sites do it, and am sure it can be built on the Amazon module, but it seems like a larger project.

Thanks!