Is it possible to query Google for shipping rates based on the shipping options set on the site? I.e. UPS, USPS, etc.?
For example, before checking out with Google Checkout, it'd be nice to show the shipping rates that come from those services before allowing them to check out.
The carrier-calculated shipping rates are determined after the shopping cart XML is posted to Google. You cannot query the rates directly, however, Google will display those rates to buyers on the Place Order page.
If you have implemented the Notification API, Google will send you a new order notification after the customer places the order. The notification will include a carrier-calculated shipping adjustment identifying the selected shipping method and the calculated shipping rate.