I run an aviation business. Sometimes its the people who offer to pay for assistance without being asked are the ones who receive it for free - they have recognised that it is how I earn my living and that what I have done for them has value.
Conversely, the ones who think I have nothing better to do than chase round after them with no thanks are more likely to get a bill ...
Turn up at the strip I fly from on my days off and you will receive all the help in the world and I wouldn't dream of charging. Turn up at the airfield on a working day - well, those are the days on which I have to earn a living and pay for the premises.