Tell them its a chore to turn up and have to leave it if they are not in.
Ask them if they can pay online, if they cant tell them its cheque or nothing then, or if its not too out of your way you could just put up with it if you want/need the work.
I have a custie that out of the way she pays online, her next door neighbour also has her windows cleaned but will only pay cash, so i only do her when i catch her in as i refuse to go collecting. That custie sometimes hides, so doesnt get done often but becaus ei ned the work i put up with it...for now