Hi

You need to perform some configuration in your Google Checkout merchant account. Here are the instructions :

1. Your site need to support HTTPS (you need to purchase an SSL certificate for your site) .

2. Login to your google checkout merchant account .

3. Access to Settings tab, then Integration 

- In the API callback URL textbox, please enter :

https://yourdomain.com/index.php?option=com_pmform&task=payment_confirm&payment_method=os_googlecheckout 

(Please remmeber to replace https://yourdomain.com to the real domain of your site) .

- In Callback contents, choose Notify as XML .

- In API version dropdown, choose version 2.

After that, everything will work as expected .

Regards,

Tuan