Tax & Shipping is one of the new required fields that must be included to the feed. The best way to pass along this information is to set up the tax and shipping calculation directly on your Google Merchant account http://www.google.com/merchants. Once you're logged in, click Setting on the left menu and then select Tax & Shipping.
If your shipping logic could not be accurately translated to Google, then Google recommends that you set up an estimated cost.
For more information on how to set up the shipping cost on your Google account,please visit the following page :
http://www.google.com/support/merchants/bin/answer.py?answer=160162