Overcharging for two months now!

I have two linked accounts with Autopage - one for my iPhone and one for my iPad. Since my iPad was due for an upgrade roughly three months ago, I decided not to upgrade since the device was still in perfect condition, and asked my local (Pongola) dealer to move me to DATA ONLY (as again, the device was paid for). My account skyrocketed from being (at most) R1700 per month for BOTH devices, to roughly R4000 per month for the last two months. This only tells me one thing - I'm getting charged out of bundle rates, i.e Autopage didn't put the iPad on DATA ONLY. They now claim that I owe them R8700 for two months - this is absurd! My local dealer throws their hands up, saying they can't do anything. I AM NOT WILLING TO PAY FOR A MISTAKE ON THEIR SIDE.