Hi Dave,
Ive been using WFP for over 10 yrs, never happend to me. Dont pay!, you will lose the others customers in the Cul as well as probably your reputation, stick to the fact that the brush with plenty of water cant
scratch the windows, they will have a real job proving otherwise (u know "not guilty until proven otherwise".
If your still unsure, then why dont you take your system chuck some sand on the windows and wash them down, I bet they wont scratch! and if they do, then the glass is more than likely lower quality.
Some Panes are produced incorrectly with the unpolished side on the inside, rather than on the outside, also there has been recently a lot of glass produced with a very soft outer surface which scratches very easy, in the future if you are worried do a window and check the outcome before continuing.