Because my work is residential and every 4wks, i sell my service on the quality of the work. Apart from likes of bird mess n severe sand storms etc, you will now have clean windows until my next visit to maintain the quality of service. When they ask about the filter water, trump it up so they know the effort (expense of your equipment) that goes on behind the scenes that enable us to do such an efficient clean when we come.
On different but sane note; when i was trad working, i had a stretch of bungalows to do. One customer always came out to check the sills n windows when i was on the next bungalow. The reason......i cleaned them really fast (mopping one window whilst blading the next), too fast for them to be cleaned properly in her opinion. Yet time and time again, i would ask her if shes found any fault. She never did, but she never tired of doing it. Think it became a challenge for her...