Angry client

Opened an account with OK furniture shop in Port Elizabeth Pier 14 and when accepting my contract I advised the consultant that was assisting me that I will only be able to make payments on the 20th as that is my pay day. Anyway I made payments on the account of more than the required installment however I couldn't see any difference but on my slip there's always an arrears balance and when I question I am advised its monies that I paid more on the account and when I enquire as to why it is not added on my account I never get a satisfactory answer. Went to OK now to pay off my account and they advised me I must pay more than the R1200 that I have already paid however on my slip capital outstanding is R45. 25 then there is this arrears amount R1618. 09- that I still need clarification as I was advised that it's monies that I paid in advance. I need proper explanation from their account department someone that actually knows what is going on with their billing as at the store they are so confused and are unable to assist. This has been going on for quite some time but I'm not really upset and ****ed off by all of this I need assistance ASAP.