Problem Description:
Google tries to contact you during finalising stage of the google transaction, but is not able to. you might get the following error message:
“We’ve made several unsuccessful attempts to send order notifications to your notification callback URL,”

Issue:
Google checkout requires you to have a SSL enabled URL to communicate back. In the case of standard modules for oscommerce, Zen Cart, CRE Loaded and osc Max, the file that handles a call back is the responsehandler.php in the googlecheckout directory. This file is accessed by google system using a SSL layer. If this file is not accessible or if there is an error in accessing the file, google sends you a mail indicating the same.

Note:The google checkout orders may still show up in the admin of Oscommerce, Zen or CRE.

Solution:Do check the expiration date. this is one of the common issues sited.
Another issue I recently encountered is with the shared SSLs. The path to the secure SSL might nt be always https://www.domainname.com

it might be something else like
https://yourdomain.webhost.com/secure/
or something similar. If the host has done the install for you or ou have done it through the control panel, do check for this information. Once done try to access the page through the secure URL. if everything went well, the page should open up without any issues ( or may be with a warning on IE stating that both secure and non-secure items are being shown ).

You can test by simply navigating to:
https://path/to/secure/SSL/googlecheckout/responsehandler.php

If the installation is correct, you should be able to see a popup coming up asking for a username/password. Please ignore the same , cos the page is already accessible.
Update the settings in the google panel as well with the new URL info and that should solve the accessibility problem.

In case you feel something should be noted here on related to the problem, mail us or put in a comment. We are glad to help you.

Cheers
Shiva