Google Checkout needs to deliver the shopper back to the merchant

The current way that you have Google Checkout set up you basically lead the shopper to a dead-end after they complete their purchase. I imagine a lot of merchants would be reticent to put Google Checkout on their site given that, because they would lose the opportunity to further develop the relationship with that shopper after they have completed the purchase through Google Checkout.

For example, merchants could suggest the shopper create a wish list, or sign up for their email newsletter, or subscribe to RSS feeds, or read their blog, or continue looking around their catalog.

You need to allow merchants to supply a "thank you for your order" URL that, when defined, Google Checkout redirects the shopper to that URL after the purchase.