Are you certain you have created a "sandbox" account, which has a different Merchant ID and Merchant Key than your "live" or "production" Google Checkout account? I made the same initial mistake (indicated in previous post on this thread). ;-)
Log into your sandbox account at https://sandbox.google.com/ to ensure everything is correctly configured for Google Checkout sandbox settings and to ensure you have copied the correct ID and key for your sandbox account.
Then double check your GCO payment mod settings within Zen Cart admin:
(Config > Modules > Payment > GoogleCheckout)
to ensure correct sandbox configuration settings for:
Merchant ID: (sandbox ID)
Merchant Key: (sandbox key)
Select Mode of Operation: https://sandbox.google.com/
--
Woody
Bookmarks