I received this about two years ago and recently came across it in my papers. I thought, "They'd enjoy this on Etiquette Hell."
I had received some raffle tickets to sell for the local first aid squad, but forgot to sell them! When I realized it I just put the tickets and cash for the amount of the tickets in an envelope and wrote "Oops! I forgot to sell these. Here's a donation." and put it in their dropbox. Somehow they figured out my PO Box (maybe it was associated with the ticket serial numbers?) and sent me the following postcard.