I often work for people of a certain ethnic origin, who always, always, always haggle about the final bill. It's a deep rooted cultural thing.
Even though you have quoted a price for a job in advance, they will always, always, always try to get a few quid knocked off when it comes to payment time.
So I've adopted the tactic of adding £10 or £20 to the estimate, knowing full well they always, always, always haggle.
I did that today - and they just paid up, no haggling!
I feel slightly guilty now......