You don't have to distract from the current project at hand though. You would do this maybe once a year. A credit would simply appear in their account, and yo

You don't have to distract from the current project at hand though. You would do this maybe once a year. A credit would simply appear in their account, and you send them an email saying something like "Thank you for supporting open source! We have taken in more money than our operating expenses require, so we're giving back to the community that gives to community software. You have $X.XX in your account to donate to any currently open project. This amount was based on how much you donated over the past N months" That would keep them coming back, and I don't think it would confuse them. I mean, people don't get confused when they get their checks from the utility company or REI. If anything I think your demographic (people who donate to fund open source software) would understand.