Address envelope only. If you bung a stamp on and people don't send you a cheque its an utter waste of money. And i collect once a month as well. Works well for me.
The receptionists used to use one when I worked at the hospital. Was good fun to use
Franking machines cost a small fortune to run. I looked into getting one, but it wasn't worth the expense.You can buy envelopes that are specially marked and the post office charge you postage on any that are delivered. I don't know any more than that.As for stamps, I use them on my envelopes. Make it as easy for customer to pay as possible. I know that others refuse to do this, but it generally works for me. And most of my round is 15 - 20 mile round trip, so fuel to go collect would cost me a lot more.