Incorrect billing for monthly installments.

When ordering online I get to choose from a 6 month interest free option or 12 month interest bearing. I chose 12 month option for my last order so that I don't have high installments for festive, but my installment is R700+. I could've just as well chose the 6 month interest free option if I'm paying the same amount still. It doesn't make sense. Are you telling me the interest is R500+??????