Billing me for money I don't owe

I have a clothing account with PnP. I have paid that account diligently since it was opened. I have always paid A LOT MORE than just my installment. But apparently I am not a valued client. PnP wants to milk me for more than what it is their share. The last invoice I received, the balance owing was R921.93. I paid R922.00. A massive 7c more than the outstanding amount, but let's not forget I paid everything (and more).

I get my latest invoice. I owe R47.50 on interest and fees - on a POSITIVE balance mind you because I did not tell reasonably intelligent people that I settled my account. I mean, the account is overpaid. I am slapped with another amount, this time R122.60 to - in their words "settle my account".

Settle WHAT exactly???????????????????????????????????????????? You have MY MONEY in your books. You owe me and not the other way around. You need to make this right asap.