Howto - it works both ways. Some customers are absolutely bloody awful.
He can give a quote or estimate, which is accepted, then once he starts the job, the customer starts adding extra requests. 'Oh, can you just do this, can you do such-and-such as you're there.' Then when they are given the bill sticking to the original quote, they moan about it and very often pay less. More often than not, the people in large houses, it has to be said.
One Christmas Eve, DP had a frantic call from someone saying they needed a job done urgently. DP was already running around like a headless chicken, but agreed to fit her in. Went absolutely out of his way to fix her problem, which he did successfully. He charged her £40 +VAT for fixing it, but she refused to pay him giving the reason "But you don't have a call-out charge". She didn't understand that a call-out charge is often £50 or more just for driving to someone and knocking on their front door. So he helped her, fixed her problem, drove there, using his petrol, time and equipment, on a day when he was already stressed and busy. And didn't get paid at all.
What on earth did she think - he just goes around doing work for people for free?
He also does, voluntarily, do 3 or 4 jobs a week for elderly people at no charge, if he's only been there for a little while. Because he's a nice chap.
So please, everyone slagging off and lumping ALL tradespeople together, remember that there are some good ones out there, and some customers can actually be utterly shitty.