Yes some people do use a letter. But I dont when they first say they want my service I use a terms and conditions leaflet, which they sign and draw to there attention important things like, payment and cancelations also if they want to cancell for one clean.
I think personally you saying that you will suspend the service after 3 cleans if they have not paid, may give them the feeling they can run you 2 cleans every time. So I would not use that I would put payment by return of post.
Roy