dpeterd,
There are plenty of scripts out there that will help with the integration of real-time shipping data; example:
http://www.hotscripts.com/Detailed/43072.htmlHowever, even if you host the sales page so you could use PHP or ASP code, I don't believe the shipping table is accessible via the API for the purpose of dynamically updating a shipping 'id' value then using the id on the sales page.
At least the API documentation and wsdl I have does not include that table.

It seems this would be a feature Infusionsoft would have to give us more data access in order to build, or add the feature into the CRM itself.
- I haven't used the shopping cart, but I'm shocked that it isn't included in the cart by default.
Mike