Fees added on a 0 balance account

I have been paying off my Jet account for the past 3 months. Every month i received a statement from Jet with +/- R100 charges stating AGENT COLLECTION FEES which i complained about and they did not respond. For the month of july my statment said i owed R617 which i have now paid in full. I get a call from these ridiculous people saying my balance was in fact r750, I HAVE A PRINTOUT FROM THE STORE AND A STATMENT SAYING I OWED R617 AND MY NEW BALANCE IS 0. The man on the phone says the additional amount is for collection fees. HOW IS THAT POSSIBLE WHEN I HAVE BEEN PAYING THESE FEES FOR 3 MONTHS AND NOW THAT MY BALANCE IS 0 IM STILL BEING CHARGED. i have had enough of this company. I have paid off my account sooner than i agreed to do and this is how they behave ! Disgusting!