What should be done should be easy. Just take off the 15% whenever someone goes about to collecting their money. If someone has $10, that's $1.50 going to you, o r in this case, Tapas. A certain limit of say a dollar might be necessary but it should not be $25.
But let's say it is, let's say this number is necessary. That in that case, why not just establish a system in which while you can build up coins, you can't send them unless it hits that quota. Or perhaps a window that pops up whenever you do send coins, giving an indication of where the creator is.
Example. I want to send 5,000 coins to someone I support. A window pops up, saying that said creator needs at least 25,000 coins before they can take out. At this point I could decide, sure I'm willing to tip them, believing it'l be easy for them to get the money later on. Maybe I'm expecting to tip them more later down the road. Or maybe I think, I don't think this will ever get to 25,000 coins (or whatever $25 worth of coins is) and hold back, deciding there's too much risk of sending in money that might not be used.