Upnormal account charges

I opened account with sheet street in February &I took towel worth R169 &my monthly installment was R50 wich I paid&I was againg charged service fee &I paid that aswell&all the other wich I wan not minding until lastmonth when they send me a text that I must pay R89 wich I did because I wanted to close the account as I don't want bad credit records I went to their branch in mokopane & they checked my account only to find out that I've been paying more than I should infact I paid double the price of the towels unaware.today again I received another text of R89.i want nothing to do with this store anymore&I want refund of my money that I paid ,what they did is day light ******* I will never recommend anyone to buy @ this store. Can I please have serious intervention on this matter,I don't want bad credit records &I don't have one &it won't start now with sheet street.. I phoned their customer service on Wednesday the service was very poor the guy keep on saying "that amount is for interest mam" I haven't used the account since February &didn't sign for any of their promotions. Thank you