I've been in the repair buisiness for 40 years.
the last 9 or so It was all "billable ' time. No maintanance contracts.
warranty gets to be a problem.. 30 days or 30 seconds or untill you hit 'start'
I would be very careful about charging, as then there is an 'implied warranty'' even when the new problem is no way related to the old one.
I'm a much better technician tyhan most hams around here. and I have much more test equipment, but I 'get a felling' about the person and their problem
before i commit to any thing that may come back to hauntme.
I can do the simple things and also will work on the others stuff
it they sit beside me and follow along and make decisions, especially when it gets to 'big bore shotgunning ' something.
eventually you can get to the SWAG method of troubleshooting.