I have never collected in 10 years, I always telephone/text the customer the night before, this way they leave the money for me, also the added bonus is they leave side gates unlocked, this way I can clean the back and earn more.
I think that at the most I am only owed about £100 each month by some who forget, but they normally pay the next time, if though after 3 cleans they still haven't paid, then they are put on hold until a cheque is recieved in the post.