Thing is, you could still have people ticking that they want a regular clean even if they want a one off. When I get new customers, I tell them the work is per calendar month, and ask if that is ok with them. Despite this, I have had a couple of instances, one recently, where after having done the 2nd clean, I had a phone call from the customer saying that they didn't want a regular service!?. You will never win, its just a hassle of the job!.