Before I started my round, I went out with a mate who's been cleaning windows for quite a few years before I started. He showed me the ropes. Trad days.
One of the best things he told me was, never collect. From the outset, make the customer aware of payment methods.Leave addressed envelope, send cheque in post, or bacs, or cash if in.
Collecting can be so hit and miss, wasting time when we could be doing something not work related.