Account doesn't seem to get done paying off

I purchased a product which was on sale and even put in my birthday gift voucher of R100, the product was R600, on account, the opening balance was just above R800, because I took it on a 12 month purchase, I've been paying since February/ March and still my balance doesn't seem to go lower, it stays ons That high amount, is it normal that businesses can ripped someone off like that?