http://checkout.google.com/ is still free! That means that small, medium, and large businesses can take advantage of using then as a merchant account through the end of 2007 with no fees. In other words if you receive an invoice from Tandem Interactive and want to pay by credit card, I will send you a Google Invoice. If that invoice is for $1000.00, Tandem receives the full $1000.00 in the bank. No fees, no transaction fees, it is awesome. I've personally dealt with setting up merchant accounts, and Google Checkout makes the process painless.
Recently, SFIMA saved over $1500.00 in fees by using Google Checkout to process the attendees for the SoFIE Awards. We used the regular check out buttons, but Google Checkout can also send email invoices.
If you have not already tried it, it is certainly worth using for the remainder of the year. Much like their Free Analytics product (Google Analytics), they should wipe out the competition by providing a superior product to the public for no cost.