So what do you put in a letter like that? Do you tell them that you are through because they are a PITA or do you use an alternate reason for dropping them? Or do you just not even bother to explain?
Price fairly but price to stay in business.
You can't charge extra because it takes you twice as long to do the job because you have crappy or outmoded equipment.
And, if they don't pay, put a shovel upside their head.