Ubercart

I'm configuring Ubercart with Google Checkout for a client. I can't seem to get Ubercart, on the Drupal side, to calculate shipping before connecting to Google Checkout. The user has to go directly from the cart to the Google Checkout domain. Once in the Google Checkout domain, it looks like I'm relegated to flat shipping rates because there's no way for Google Checkout to know the weight or size of the package. What I want is for Ubercart to calculate the shipping and add that cost to the subtotal before handing off the transaction to Google Checkout. Alternately, I want *some* way for Google Checkout to use carrier calculated shipping from the USPS. Can someone help me out? Tell me how to accomplish this without PHP programming?Thanks,--Marshall