Why should there be a charge for signing a piece of paper? I do not charge, I just ask that they supply a stamped addressed envelope.
Reaming pilots at every opportunity even if it is for a fiver is what is killing GA. All of these tenners or 'even fivers' add up. Flying is expensive and every pilot we put off with rip off fees is one less person to put revenue in the coffers for fuel, landings, food, pilot supplies etc. Cutting noses off to spite faces.
Money is tight and ripping them off to sign a piece of paper that takes 2 minutes is just obscene.
I prefer to think that if you don't leave people feeling ripped off they come back for further training, license revalidation flights etc. Seems to work.