David, that's really good and if this were backed up with your "own" Business ID card, would surely put the customers mind at rest that they are getting someone reputable to clean their windows for them.
Just a hint for others, if the potential customer asks do you want payment in cash? Answer this with, "you can pay in cash, by cheque or on-line if you wish, as we declare all our earnings, we don't mind which method you choose"
We all experience the "my old WC only charged this much", but just remember they may just be "downing" the real price themselves, thinking that you are desparate for the work! Another way of dealing with it, is that if you get the "used to be £2.50" scenario, then just politely answer, "in all honesty Mrs Customer (or Mr) my price will be higher than that, would you still like me to provide you with a quotation?" Keep away from mentioning "dolers" or such like. Don't undervalue your own work!
