Wasn't there a study about childcare facilities where, after a fee was introduced, late pickups actually increased? Putting a specific price on things can actually make people behave worse than just "be reasonable, please."

We left a bucket out while we were out with the kids. No issue. Had too much at the end of the night, and had to tell the last stragglers to just take handfuls when we got back. Maybe that's a good, quick temperature check on whether your area is high-trust/low-trust.