Google Checkout Level 2 for OsCommerce

[Alex]

This contribution will add a full support of a Google Checkout service to your Oscommerce store.
While using Google checkout at your OsCommerce store you will be able to allow your customers to buy from you quickly and securely, using a single username and password without any account creation.

Benefits:
  -Google Checkout for OsCommerce will run on any PHP4 or PHP5 enabled web-server
  -No requirements for using libXML and libCURL installed.
  -Full google checkout sandbox support
  -Full shipping support using merchant-calculations google checkout shipping
  -Full tax support over merchant-calculations in google checkout using taxes from OsCommerce
  -Basic shipping modules like flat-rate and per-item shipping supported
  -Basic USA shipping modules: UPS, USPS, FEDEX with real-time quotes are supported
  -Order management inside OsCommerce Administration area: orders placed via Google Checkout will appear in your OsCommerce admin, and you will be able to process them (Charge, Refund, Ship, Track etc) from your usual OsCommerce orders page

All support for this contribution will be performed here.

[Alex]

If you ever have encountered an error in your integration center like this:

We encountered an error trying to access your server at https://www.yourdomain.net/your_store_dir/google_shipping.php -- the error we got is: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target


The error just means that, as google says "What this usually means is that the server is using a test certificate (possibly generated using keytool) rather than a certificate from a well known commercial Certification Authority such as Verisign or GoDaddy."
and
"Google Checkout will only communicate with a server running 128-bit SSLv3 or TLS, and the SSL certificate must be issued by one of the Certificate Authorities in the Java 1.5 Trust Store."

Please make sure you are using a valid ssl certificate.
If yes, you could contact google checkout team:

"If you're still having trouble receiving callbacks, please contact us
via our Help Center (http://checkout.google.com/support/sell/bin/request.py) and provide
your API callback URL so we can double check your SSL certificate."

You could also refer here regarding this issue (google checkout answers):
https://checkout.google.com/support/sell/bin/answer.py?answer=42923&topic=8671

Please ask your hosting provider regarding SSL certificate if you dont know why this
is happening.