flying mallet , lol I like that.
I know I lack tact, but seriously some guys on here take the biscuit.
About the stamps, yes you put stamps on the envelopes, 2nd class ones are fine. The reason for this is that it encourages them to pay faster (one less barrier - cant find a stamp!) also they wont throw an envelope out that has a stamp on it because it has obvious value.
Believe me, the 22pence or whatever is well worth it. You dont have the bother of collecting and you'd pay that in petrol anyway. Also, beleive it or not, customers dont like you bothering them in the evening for money, they will much prefer to post you a cheque.