Dear Mr. Manager!
I am writing because of an unpleasant incident that happened at your car wash.
Although I followed the car wash instructions, the machine damaged my vehicle, torn off one of the side mirrors and scratched the paint.
I went to the cashier to complain, but the surprised cashier couldn't help it.
I would like to receive a refund for the damage caused to your car wash to compensate for the damage. I will be grateful for a response as soon as possible.
Yours sincerely (twoje imię).