Hi Henrik,
Over the years I have integrated with a number of payment gateways and it is without fail, almost always a huge pain in the butt. Last year I created a working integration with Authorize.net and an AwareIM application but it I like this approach a lot better.
When working through the integration with Authorize.net I originally tried to integrate with several payment gateways to try to implement recurring billing but always hit a brick wall. In many cases, it was due to the limitations of AwareIM to discover anything other than the most simplistic wsdl files via soap. (I am interested to test this approach again now that they have improved the discover functionality.)
Anyway, after giving up on that approach last year, I finally ended up creating a hosted custom php integration with Authorize.net and then communicating with those php pages directly from AwareIM to post and get the various responses to/from them. In essence, I created a custom connector/bridge that you have mentioned above. (Incidentally, for users with php or asp etc... experience, this type of interaction between custom php/asp pages and AwareIM is one alternative to custom java plugins for those of us that don't want to become java programmers.)
I have spent a few minutes checking out Formstack and I really like that idea. I have created a trial account there and dabbled with it and it seems to me that you could post the data from your AwareIM app directly to a form confirmation page at Formstack and show that page inside your AwareIM app. Formstack would handle the integrations with the payment gateways as per your referenced article and the user could interact with the embedded page in your app and receive the visual feedback from that page. Formstack could push the submission results back directly to your app for further processing and updating of the payment results in your app.
In summary, I'm not saying that payment gateway integration's are not possible natively with AwareIM particularly with the new Web Services improvements. However, for situations where this is not practical for whatever reason or when wanting to integrate with multiple payment gateways, Formstack would appear to be a viable and cost effective option.
Cheers,
Pete