I purchased the Add a Tip plug-in from Thompson Web Design--recommended by paul2009 above--and modified text so people could add an optional donation at the time they purchased a ticket for a virtual musical event. I had some challenges setting it up and Bradley Thompson was extremely helpful, responding promptly to e-mails. If you're selling a service product--and don't have to contend with the tax issues mentioned by aladdinsane--I recommend the plug-in.