3months cash deal and now interest has been charg

Good Day<br> I ordered a bedding set from homechoice in the month of July to the value of R3950.00. the agreement was a payment over 3 months cash deal. I then paid the account off within 2 months instead of 3 and I have been charged an additional R700 on my account for what I don't really know. Each time I do enquire from the call centre I am unable to get a response from the staff and neither the manager because miraculously they also don't know. I keep calling trying to solve the issue but to no avail and the interest keeps piling up.. I have even requested for the voice recording to be checked in order for us to hear what exactly our contract entailed but yet again nothing is being said. Please their service is poor and it is nit the first I have trouble with this company. I am very disappointed as I am a regular customer who always pays my account in cash and yet I am being treated like this. I would just like the matter solved and my name cleared.