If Im ever getting a "small" amount of money from a customer and they say anything, i always say yeah but think about it 100 people who owe us £10 thats a lot of money, and we do some way bigger houses than this for a lot more dosh!
Also when you get the people who say they bet they are the worst paying customer, expecting you to say no, there are far worse payers, to make them feel better, just say, no sorry you are one of the worst!!!