No, really, it's up to you
The rock band Radiohead is putting this phenomenon to the test: You can buy the download of their latest album for whatever price you choose. You can pay as little a penny or, they hope, much more. The whole price you pay is, in essence, a tip (but paid before the service is rendered).
Since we economists don't understand tipping, we can't really say whether this new scheme will work. But if it does, during my next ec 10 class, I will put a hat next to the lectern.