it takes a while BUT the customers get the idea in the end
i knock on the door after the clean and collect, if no1 is in, i just leave a note with
Your windows were cleaned today
Sorry we missed you for collection, we can either catch you next time or you can send a cheque to : address
or you can pay online using Nochex or paypal
it works ok, some leave it for a clean or 2, some of the bigger accounts even leave it 3 cleans, then i have a bumper payday