Just tried the tool provided by GoogleCheckout to test the response handlier

responsehandler_test_v1.2.php:

HTTP/1.1 400
Date: Wed, 26 Mar 2008 22:23:05 GMT
Server: Apache
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Pragma: no-cache
X-Powered-By: PHP/4.3.11
Set-Cookie: zenid=b991bb6de2f0ee66f6d66a9fd5cf8e5c; path=/; domain=www.austincotton.com
Connection: close
Transfer-Encoding: chunked
Content-Type: text/html; charset=iso-8859-1

Invalid or not supported Message

Additionally, the entire site doesn't work after Installing GoogleCheckout from the Admin.

Code:
Warning: Variable passed to each() is not an array or object in /home/content/a/u/s/austincotton/html/includes/classes/db/mysql/query_factory.php on line 114

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/content/a/u/s/austincotton/html/includes/classes/db/mysql/query_factory.php:114) in /home/content/a/u/s/austincotton/html/includes/functions/sessions.php on line 108

Warning: Cannot modify header information - headers already sent by (output started at /home/content/a/u/s/austincotton/html/includes/classes/db/mysql/query_factory.php:114) in /home/content/a/u/s/austincotton/html/includes/init_includes/init_templates.php on line 78