why are you knocking on their door in four weeks time, explain that's its a monthly service and just clean them, if they want a one off they can be charged double.
you have to think of your business it must be sustainable, window cleaning is all about regular work and regular reliable customers, you will always use more water on first cleans, but far less on regualar, what s the point in purifying water at all and providing a service in which windows air dry to perfect results if you finish the job then get the trad equipment out and blade the windows, not only does that show not a lot of confidence in the system customers will come to expect it. water fed pole is made to leave windows wet trust the system, you will work so much faster this way, which at the end of the day earns your business more money