I've used Paypal for payment processing in the past - they charge just over 3% as a transaction fee - this covers the cost for their services and the credit card transaction fees. They've got tons of features, and its easy to add buttons to your website and start selling things, including memberships, subscriptions, and download-able or shippable goods.

Google payments has these things in it's favor:

• No fees through the end of the year
• No fees for non profits until 2009 (they just started working with non-profits)
  
• Automatic direct-deposit of funds within 48 hours
• They won't work with porn sites.
  

They don't do re-curring payments, and selling subscriptions with Google is right out of the question, but especially for rendered services and donations for non-profits, it's a great new option.