Make sure the following conditions are met, as described in the install.txt
If the 'Merchant Calculated Shipping' is set to 'Yes' and
some of your products have the 'Requires Shipping' set to 'Yes' then
at least one of the supported OpenCart shipping modules should be enabled.
The Google Checkout module has been tested with the following standard shipping modules:
- Free Shipping
- Flat Rate
- Per Item Rate
And it has also been tested with the following contribution:
- Zone (Available from http://www.mhccorp.com/downloads-opencart.shtml
Make sure all of the enabled shipping modules which support a tax class
use the same tax class.
Make sure the product's tax rules, if there, are compatible with Google Checkout.
In particular, tax rules are only allowed for the UK or USA.
If a shipping method specifies a UK-based geo zone make sure it includes
all UK counties.
If the 'System > Settings > Option > Stock Checkout' is set to 'No' then
whenever the customer tries to add an out-of-stock item to the cart
it won't be possible to do the fast checkout with Google Checkout.
Use Google sandbox seller and buyer accounts for testing before going live.
More details on sandbox accounts can be found at
http://code.google.com/apis/checkout/de ... x_Accounts
Also, check your system/logs/error.txt and system/logs/googleerror.log for any error messages as to why you can't check out your items.