Account still incurring interest after settlement

I would like to state my disappointment with RCS, as in May I requested for a settlement amount and paid the amount the same day, even sending them the proof of payment. At month end i received their statement showing a credit balance after my payment and they then added interest. They did not close my account I then email to inquire about this with no response. Now i am getting sms that say I have an outstanding balance that I have to settle. Please educate me RCS, what is the point an a settlement if we settle our bills but after we still owe you? Please close this account, its not fair that I worked so hard to make sure I pay you off and you are still charging me interest on a settled account.