I've never provided stamps.
Old gaffer used to.
I print a little 'don't forget your stamp' in a stamp sized box on the envelope.
Had a few still forget and end up having to pay the £1 at the post office.
Now if I get a slip from the postie I go to the post office and ask to see the envelope before I pay.
If it's one of mine I don't accept it.
Payment not received and add outstanding to customers bill.
Very, very few & far between now though.