Google Checkout is moving from a standard fee schedule to a new tiered pricing model where rates decrease as merchants process more transactions through Checkout, begining May 5:

• Rates will vary depending on a merchant's monthly sales processed through Checkout. For more details about pricing changes, visit U.S. fees and U.K. fees pages.
• merchants can qualify for rates as low as 1.9% + $0.30 per transaction in the U.S., and 1.4% + £0.20 per transaction in the U.K.
• Will discontinue the AdWords free transaction processing promotion. Google Grants recipients will still be eligible for free donation processing until 2010.