I was thinking of offering a warranty on the repair. Like 6 months.
-warranty is a tough one I've learned. I used to repair a lot of pro audio equipment. Some years over 400 repairs. Some people are sane and normal. But there are people with other ideas who will claim warranty for each and every problem it develops after you worked on their equipment.
-Document everything about the equipment and deliver that to your customer. Cosmetic condition, supplied cables,... Because the not so sane people will claim they delivered whichever usually expensive option and now it needs replacement at your cost. Or that scratch wasn't there... bla bla, I will not pay your invoice. (But I will return next time because you are actually good at what you do)
-Document everything if you notice any damage has been done by someone else in an attempt to repair the equipment.
-Invoices: again the same thing. Normal people pay your invoice within days. But on some you will have to spend a lot of time in order to get your invoices paid. And if you refuse to take on new repairs, they start to pay all open invoices a few days before they reappear. The remedy for those is overcharging for the time you will have spend on getting your money. Payment at take out is an other option. They will hate you for that. I must have been doing my job pretty well because they returned anyhow.
The nutters:
Sir, the microphone you repaired makes a strange clicking sound at maximum gain (over 60 dB). It never did that and I demand a new one and right now because I have work to do with this microphone. (Those extremely sensitive microphones are pretty expensive and blowing onto the membrane is enough to kill them) Eventually he brought it back and after some tests nothing could be found but at his location it made a clicking sound. After some more questions it became apparent that the clicking noise had a period of one second. What a coincidence. Is it by any chance the sound of a mechanical clock? It turned out to be his own watch... He never apologized for the screaming, shouting. Because well why would you?
The funny ones: I was once asked to repair an amplifier from an expensive disco hair saloon in Paris. Complaint: it shuts down and the red light are on (over temperature in this case). The amplifier was completely filled with hair shavings and hair varnish front to back. I called them and after introducing the problem I said I refused to work on it unless they would shave it first. But they didn't get the joke. I can hear the person on the phone having a very confusing conversation about my question with two other people. After a few minutes they decided they wouldn't know how to shave an amplifier.