Absolute frustration

Two months ago I opened a 6 month account at one of their shops (Frazers) to buy a purse that was on sale. I was going to pay it in two installments. I paid the first installment and last month I paid the last installment which is the balance of R399. I made an EFT payment but my reference which is my card number was incorrect. The payment went through but obviously they could not allocate my R400 which I paid because they had no account reference number. I am now getting emails and sms's for the arrears amount. I have sent proof of payment 6 times since Monday and up to now they have not received one of my mails. I have checked and rechecked their email addresses but they still do not get it. I'm now convinced that this is just a way to add interest to a paid account. I have never encountered such incompetence. I only bought one item and had the VIP card for just over two month but I cut it up this morning.