I've done it a few times, just handed them the bill of the consumables/replacement parts. Most had the decency to thank me profusely me and douse me in alcohol
What also works well for me is just invite the person in question over on a nice spring/summer evening, and wrench while chatting and drinking away.
I once fixed a beautiful Gazelle for a very talented singer - she showed up with a bottle of wine, got behind the piano and sang to me while I trued the wheels and adjusted the brakes!