Unnecessary Charges On Paid Up Bills.

On 1/12/2014 I paid R350.00 to RCS on my Game account. I then found out that there's still an amount of R38.47 outstanding. On the 3/12/2014 I paid R40 to finish all outstanding debt on this account, and saw that my account now stands at -R1.53. On 21/02/2015 I received a message from Game, that states, my account is in areas with R63.14. I phoned them (RCS) on 23/02/2015 and was informed that my outstanding balance was R83 something. This was because I never phoned for a settlement amount. I was charged interest on an account that I have overpaid with R1.53, plus other customer protection fees. On 2/03/2015 I phoned again for statements, and saw there were charges added while I owed them nothing. This is ****************. I am suppose to pay R85.88 now for not informing them that I already paid more than what I was supposed to pay. How can you pay interest on a paid up account?