Extra charges

I have an account with homechoice, I bought blankets but 2get 1 free ×2 and my was R5383.18 . Am supposed to pay R953 a month and am paying R1000 a month, and I payed for 3months now and they send me statement stating am owing them R3719.62 .they send the statement on the 12October after 3days they call me that am owing R2700 and something is for what? Cos oven pay extra R45 but now 2700 is for what ?if I ask the agent she said I must call them to check but am owing that money. If you minus R3000 from R5383 won't be R2700 like they said. i even pay extra money they can't rob me like that pls help