1. Go to the Settings → Payments section and, in the subsection Merchant Setup tick the checkbox Accept payments via Google Checkout.

2. Provide your Google Checkout Merchant ID and Merchant Key, and click Save.

3. Make sure the checkbox Enable payments for this form at page top is enabled. This is all you need to do on the 123ContactForm side.

4. Log in to your Google Checkout account and go to Settings → Integration.

5. In the API callback URL field, use the following URL:
http://www.123contactform.com/googleipn.php?mid=XXX&mkey=YYY
Replace XXX with your Merchant ID and YYY with your Merchant Key.

6. (optional) If you want to test the integration using Sandbox, you need one extra parameter: http://www.123contactform.com/googleipn.php?mid=XXX&mkey=YYY&servertype=sandbox