I always phone the night before, and yes you do get the occasional "not this time" but this is far outwayed by the side gate being unlocked, so the back can be done and i earn more.
if customers keep on doing the "not this time", well I just forget to phone them again, when they phone, I explain That I am a regular window cleaner, who needs to earn a weekly living and they are not my type of customer.
If you don't put a stamp on a envelope then you will be waiting forever for payment.