When we first started out some 6 years ago, we always left a envelope with a 2nd class stamp on, this would only be left after the 2nd clean was owed from 4 weekly and every clean for 8 weekly customers.
As our business grew we found we were leaving a considerable number of stamped envelopes and found quite a few were not really being used to pay us. They would either pay us online, see us while out and pay etc.
So for the last year or so we now only leave an envelope and the customer pays postage. It has not in any way slowed customers from sending cheques, in fact some now insure they either are in when we call or leave money somewhere to collect, I guess this is to save them a stamp!
We did also try not leaving an envelope at all, this did slow payment returns.
Steve G