When I recently changed from traditional I just turned up and started. No pre notification.
If they were in, I briefly explained when I knocked the door after cleaning and if they were out I explained when I went collecting.
I only lost 2 customers out of about 250, and they didn't give me a chance to even start or explain. I just walked away.
Had about 3 others who were reticent based on their previous experiences with other window cleaners.
I just said I'll clean them and if they're not happy with results , no charge.
I had worried about printing off a sheet to explain the in and outs of pure water but couldn't be bothered.
'As long as the windows are cleaned' seemed the most common response.