Barry Schwartz has a step-by-step overview of how to use Google Payments.
It seems like a nice little income stream for Google…
Google Base sellers who accept payments through Google will be charged 2.5% plus $.25 per transaction. If you incur a chargeback, you’ll be charged $10.
Fees will be deducted from your account when you charge your order. If you refund an order, Google will return the variable portion of the transaction fee (2.5%) to you, but will keep the flat portion of the fee ($.25).
More details on accepting payments.