Do we have mug written on our foreheads sometimes??
The only times I've been taken for a 'mug' is when I've put myself in such a position to be charged as such. Usually when, due to inexperience or because I've hurried a quote, I've under quoted.
Now, I just state my price and give them a frequency suggestion (if I can) and see what happens. If they don't accept, then I'm fine with that, if they do accept then I'm fine with that.
Of course any customer could cancel after a first clean, and I rarely charge an initial cleaning charge.