I had a "customer" complain about a labour charge on repairing her tenants boiler a few weeks ago.
She called me after I emailed her chasing payment, at which point she spouts off about the unfair labour charge for diagnosing and repairing the boiler the in same day, with no delay. I charged her £25 per hour, instead of £35, because she works for an estate agent that occasionally give me work. I told her I'd email a response as I was in car with missus on speakerphone at the time.
I knocked another fiver off the hourly rate and basically said, thanks for bleeding me dry - Out of that £20 per hour, I have to pay for GS, ACS, PLI, Van, Van insurance, Income tax and all my other business expenditure, then hopefully at the end of the month, I'll be able to pay for my mortgage, bills and put food on the table while my missus is on state maternity pay.
She paid the original invoice price.
TBF most of my custs are okay, few late payers but not many complainers/ rude people. I've learnt to avoid them.