One of the easiest ways to get paid for something is to to offer commercial support once something shifts to 'end of life' for community support. Can't get off of an old busted version - write a check for continued support! This sort of thing is reasonably easy to get through the accounting process.

Leaving a tip, paying for something free, trying to send money to a group that our infrastructure absolutely relies on - stupid hard. Commercial support, on the other hand, is very understandable to the bean counters.