Why not make it even harder for yourself .
When you post the slip after each clean , give all custys that day 3 dates to choose there next clean ..
Ask them to text you there preferred date , then send them all a letter with the date that got the most votes . With a return slip and envelope with more choices .
1 - yes. Please. , that was the date we chose anyway
2 - well ok , it wasn't our preferred date but we will go along with it
3 miss the next clean , we never get our preferred date
P.s If the weather is extremely bad on the agreed date , you will get a text that evening with an alternative date .
Pps. . Please state if you prefer a morning clean or an afternoon clean .
4- DONT clean our windows again , the wife thinks your a bit odd .