enigma1 - 12:23 pm on Sep 27, 2010 (gmt 0)
Although old post, from my experience depends on the cart. You should be able to do level-2 integration with GC and so you can update the orders as the google server notifies you of the order progress. The concept is the same with all the popular payment processors where the client needs to login externally. Paypal instant payment has a similar process.