I leave virtually every customer with an invoice and a stamped addressed envelope unless they happen to be in (not many) I always use a first class stamp....I think that a second class stamp gives the customer the wrong impression (like you arn't that bothered when you get your money...it devalues the business)
It was a bit rough for the first couple of weeks, but now the cheques roll in to my PO Box on a daily basis....if I havn't been paid by the time they are due again (monthly) then they get a phone call or a note to tell them they won't be cleaned again until the account is clear (in nicer terms than that...lol)....seems to work ok...I couldn't stand the idea of having to go back on the evening to pick up money...it'd be like going to work twice. Anyway, it would be another 25 mile round trip...and that's the nearest ones ! Most are fine, and pay up within a few days. One even sent me a Christmas card and tip with the payment, bless 'er.