I think for a smaller shop / startup the easiest way to go would be to have users enter their own google adsense client number and use that a certain portion of

I think for a smaller shop / startup the easiest way to go would be to have users enter their own google adsense client number and use that a certain portion of the time. By making them sign up for Google you get G to do the footwork of verifying they are eligible, and managing payments...etc