Google checkout uses a three step process much like express checkout or proTX. It is pretty complex.
My opinion is that you could probably make an implementation for yourself pretty easily if you are good with Google Checkout... knowledge of our cart probably won't help much since that was never the hard part at least from our perspective.
Even a user guide may be difficult without configuration settings through a UI since there will be a lot to configure. We did a recent survey of the customer base around that issue. About 35% of our users were interested in the feature, but that is actually pretty low compared to other features listed. After all people could claim they wanted everything since cost was never a factor in the survey, so those numbers are always skewed toward "I want everything". We calculated it to be one of the more expensive from an engineering perspective because of the complicated process and how many interfaces it would effect to implement properly.
I'm not sure how you are proposing adding it. It could be added to powerstore probably relatively easily. Adding it to the extensions is a lot of work and a configuration guide in my opinion would be hard for a webassist user to follow and they would be put off if WebAssist promoted it with complex configuration instructions rather than a full UI.
We calculated that it would cost us somewhere around $20K to do the feature right and just didn't think it would move the needle that much based on the survey considering limited resources and opportunity cost of not working on other projects and features it just doesn't make sense for us yet.
We are actually in discussion with several payment gateways that aren't currently implemented and encouraging them to fund our development in order to get it into the tool. This approach has worked for us in the past and allows us to add features without having to depend on our users to pay for them, which is kind of what the economics around this feature necessitate at the moment.
