Strange to clean for nothing. IMO it lowers the worth of your service. It looks like "Yes, I clean your windows for 20 quid, but I could just as well do the job for free"
It's like buying a sofa for 1000 quid, then seeing it in a sale for 400 quid. I don't clean a single pane of glass for free. Wonder if I've lost customers because of that. Who knows, who cares? Like Daz says, "there is always another customer just around the corner"