Since this comes up quite often over in edublogs land, maybe an extra step of checking which blog gets the supporter upgrade if the upgrader has more than a single blog would be a good idea. :slight_smile:
edit: Or at least a "You are purchasing an upgrade for http://xyz.wpmu.tld blog. If this is not the blog you want to upgrade, here's a list of your other blogs. Please pick one of them." notice on the supporter page.