Hi Lighthouse,
From NZ too

May I suggest
a) make sure that on all statements you have something to the effect that 5% interest will be calculated monthly on all accounts unpaid by the 20th of the month following (totally legal, and will hold up in court)

you make sure that anyone wanting to order on account has signed away their life to you on paper
c) get knowledgable with debt collection - alot of compaines don't actually intend to defraud you, but they were expecting a cheque in on the 20th of the month that never arrived, which means they can't pay the bills they were going to pay - nasty trickle down affect usually caused by someone along the trail needing to be shoot for de-frauding someone.
Phone calls are more effective than just another "please pay this now" letter. Personal visit are even better - but you've got to way the cost of labour with the cost of the debt (although, sometimes it really is just about causing the buggas to squirm).
upshot - totally agree - compaines that don't pay up when you've done them the favour of allowing them to pay on account get right up my sneezer.
Dragon