LWC,
I know exactly what you are on about, I have had it a few times myself over the years.
A woman argued years ago that she didn't owe me £1.80 x 3 = £5.40, I stood my ground for 20 minutes trying not to argue, then when she eventually got her money out to pay, I said keep it love because you clearly need it more than me because I earn £ per hour
I only said it, to really p--- her off, I walked away without her money and she was going bananas, ha ha ha
I did it for the principle, as they look down their noses at window cleaners usually, and I thought I would put her in her place