I always word my quotes to the detail, so if there is any comeback about not doing a certain bit of the job that hasn't been on the quote, I have them by the balls.
For example, Mr Smith wants you to tile his bathroom wall in a splashback style.
Then, on the day you do exactly that. But then he decides he wants it top to bottom. You tell him its going to cost more, but he then gives it the big one and says he won't pay you unless you do it.
If you have sent him a written quote and you have a copy of this, he hasn't a leg to stand on. If its just a peice of paper you wrote on the day whilst quoting in his kitchen, it might be another story...... 🙂