I've liked this idea since you first showed people over on TalkFreelance. You seem to be edging over towards the chartiable side of things, yet your site only mentions that fact. Perhaps a page or two detailing what exactly the charity side of things entails would be useful, if only for transparency. Also, it might be an idea to print the charity number on all pages (in the footer).

As for the script itself, I had fun click-botting it back before you made changes, then clicking thousands of times legitimately in previous iterations of the site. Then it fell off of my radar until reading this topic. Hopefully you've ironed out the problems with people trying to game the system.