It depends on how you get the new customer, if it is by refferal and within your work area, then it is unlikely that you will lose them, unless you make a right mess of the windows. Having said that, I have just had a right nana, wanted to know why I hadn't cleaned the conservatory roof , I only charged her £12.00 for the whole house and conservatory.
But if it is someone that has looked you up from an advert, then you have to be much more careful, and qualify them to the best of your ability. After all it is your time they are after, so be careful who you take on.