This contribution will enable your orders to be processed through the regular checkout process (allowing you to use whichever modules you already have in place; i.e. Gift Cert, coupons, FedEx, UPS, free shipping flags etc.) before the order is passed on to Google, similar to the way it is done with PayPal. if you run a Google Adwords campaign and you use this Google Checkout IPN as "an alternative payment processor" instead of the checkout process preferred by Google (Google Checkout module for osCommerce http://www.oscommerce.com/community/contributions,4556/category,1/search,google), Google will not display a Google button (badge) in your ad. However, you can still take advantage of the no processing fees! Since not everyone runs an Adword campaign, this module might just satisfy your needs.
NOTE: I do not take or want any credit for this module as it was NOT written by me. This is just an old module that simply works with your existing osCommerce without much modification.