"Unsigned carts" error may occur, if you get this you need to configure your google account.
There is more details about this error at this link: http://checkout.google.com/support/sell/bin/answer.py?hl=en&answer=57521
To change your settings:
|1.||Sign in to Google Checkout. |
|2.||Click the Settings tab. |
|3.||Click Integration center. |
|4.||Under 'Shopping cart post security,' uncheck the 'For extra security, my company will only post digitally signed XML shopping carts. (Google should reject all others)' option. |