This would really bother me too,
its not the money.. not at all.
its the fact they are speaking to you in such a despicable way, means she has absolutely no respect for you whatsoever...
This lady is a prime example of the public perception, of window cleaners.
I would sit down, and write... no... type her a well set out letter, explaining how shocked and embarrased you were to be spoken to, in such a distasteful manner... when you have served her well for.... however long.
Explain that you run a successful business, and that you have high standards of which your reputation is based....
Explain that her neighbours will be questioned, due to her not being satisfied....to make sure they are happy, as you recieved abuse and a complaint.... so you are following up her comments.. as customer satisfaction and relations are high on your agenda.
Put the ball in her court.... the guilt back on her...
And put £3.50 in the envelope, saying the money for the clean before is also enclosed.
A statement that is telling her you dont need her money...
reverse psychology, sometimes works wonders.
Try it.
.... if nothing else.... you will feel better!
.