I want a refund

Hi I bought furniture from Lewis Stores Orkney branch. I paid my account within 4-5 months instead of 36months. But I was charged so much interest on the account. Even after paying the total initial credit amount, I was told I owed R3000+. I paid that amount but only received less than half of the credit amount remaining. I paid my account timeously every month. Why so much interest? I feel my I should get a refund of more than R275 and a clear explanation. Coxz at the branch am not getting any satisfactory answers after I paid so much money.