I detected that when you add the file "YOUR_ZENCART_FOLDER/includes/templates/YOUR_TEMPLATE/templates/tpl_shopping_cart_default.php"

The option payment goes OUT of the Checkout page. So the customers see two buttons: "Checkout" and "Google Checkout". Big mistake because the payments options are precisely into the Checkout page.

The customer choose "Google checkout" option and pay but the administrator never receive the order because he paid without register into the Zencart client's database.

I tried this method one year ago and now I tried again but they continue with the same problem. Who will choose Google Checkout module if they never fix this problem.