Handed over on upfront paid account! Account was settled in full.

Been handed over to collections for R150 even though i have paid this account in advance. Paid R3200 in advance (even though this account was on a monthly debitorder which was never stop. So even R150 outstanding amount could still have been deducted without any issues). Now getting told it is for service and insurance fees!! What are you insuring or servicing if the account is R0??????? Ridiculous!! Being told i first need to get a settlement amount before last payment! What settlement if paid up to the outstanding cent??????