Don't let 'em get to you is the best advice. For 6 quid walk away and never go back.
I had a bloke phone me once, said his w/cleaner had let him down, I went, quoted him £9. He complained and said the last guy only charged £7. I said where is he now? He mumbled that I should do them anyway, to be honest I should have quoted more, but he went in. He came out a couple of minutes later slapped the money in my hand and said he had to go out, left me to finish, and asked if I would ring before coming next time so that it didn't inconvenience him!!
So I got a little mad, finished the job and wrote on his ticket "Keep your extra £2 I don't overcharge, and all my customers are satisfied. Don't call me again".
Guess what? He didn't. Made me feel much better than £2 ever could.
Don't worry be happy
Pj