From Wiki

Introduction

Implementation via Google Checkout will require a slightly different implementation as the checkout system doesn't allow for auto redirect to the confirmation page.

What do I do?

With regards to the Google checkout source code, the URL attribute of the <parameterized-url> tag should contain our image pixel code as the web beacon (see the blue URL in the example below).

Please note only the image pixel can be used with Google checkout tracking and you should populate the variables in the image pixel with the relevant data for that transaction.

You will then have to map our parameters in the image pixel code to the Google checkout parameters – as in the template example below:

(If another commission group code is used make sure this is passed in instead of default)

There is also further information which can be viewed here