For those who opt to choose google checkout I found this out after my shopping cart was persistantly being rejected by Google checkout and giving me a fault notification that I was missing an xml document

After a few frustrating hours and reading through most of their help documents I found the solution in the steps mentioned below and solved the problem. Now the google checkout is now working fine

I hope this information helps others who may encounter this problem.
It is not enough to just add your Merchant ID and Merchant Key in under the CC payment method option in the shopping cart. You also have to go into your google checkout account and then follow these steps:
1. Click the Settings tab.
2. Click Integration center (left hand side)
3. 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.
4. Click Save.
Without doing this the google checkout will not work. Of course you aslo have to make sure you have verified your bank account with google checkout and that your google checkout is active.