Possible Error #1:

When I click Checkout using the Google Checkout payment option, I receive this error message:

Google Checkout has returned an error!
Ack: error
Message:
Post:

The Fix: You'll get this error message if there is a problem with your Google Checkout account. Double-check that you have added the correct Merchant ID and Key under the Payment Options tab. It would also be a good idea to log into your Google Checkout account and verify that it is active.

Possible Error #2:

When I click Checkout using the Google Checkout payment option, I receive this error message:

Google Checkout has returned an error!
Ack: error
Message: The currency used in the cart must match the currency of the seller account. You supplied a cart with GBP and the seller account is associated with USD.
 

The Fix: This error occurs when your default currency type does not match the currency set in your Google account. Your Google Checkout and Shopping Cart currency values must be the same. Go to the Payment tab in Shopping Cart Creator and double-check that you are using the correct default currency.