jet account accumulating balance though it was settled and closed

I had a jet account that I thought it was closed as I have settle all the outstanding balance. last year June month I went to jet store to pay the outstanding amount when I got to the store the amount to settle had increased by R200 but yet the lady that assisted me said the account had been closed, I did not complain I paid the amount given to it was about R1900, I asked the lady for a settlement letter they told it will be sent with in 48 hrs, I have never received that letter till today, I was checking my credit score last year its showed that I have an outstanding amount of R374 which can not explain where it comes from I have been waiting for them to fix my account they have not until today. i am so fed up with them.