activate magento shipping cart rules for google checkout

Magento and google checkout are already integrated, and the shipping methods in magento apply for google checkout too, the thing is that if I apply a shopping cart rule for example saying that if the subtotal price of a product is greater than USD 20 then the shipping is "free shipping" and if it s less than USD20 the shiiping is "flat rate" google checkout doesnt apply it. It is working for paypal but for google checkout doesnt: for example if both shipping methods: flat rate and free shipping are activate in GOOGLE API, when redirects to google checkout to make the payment it has the option to selected either one of both without applying the rule... The easistet way to solve this issue is to use googles shipping set up instead of magentos and apply the rules directly from google checout settings, but is not taking it either... I need to solve this issue either way... Thanks in advance...