Ugly Duckling says:
You can gun this duck down only once
but save my butt and I will trust you for a lifetime!
At no point is the urge for instant gratification bigger than in situations where customers are at your mercy. And at no time is it more detrimental to your best interests to take advantage of this. Not only will your victim feel cheated (and rightly so), but you have just blown a great opportunity for prolonged and profitable 'easy' business.