Incorrect DPP Billing

I requested that the decoder that was delivered to me be converted to a payment plan and a refund of R149 be refunded to me however the opposite was done i wasn't refunded but actually billed more by R779. I am tired of having to fight with DSTV to do things correctly and the funny thing is as soon as the error is resolved someone or the system goes back into the account and makes things even worse, this month also i will be paying more than i should pay. I am now regretting choosing to pay via debit order as i am always at a disadvantage with DSTV , if only they would pay me my cellphone bill that i incur to correct billing errors every week and actually they are supposed to pay me interest for the period that they have had my money that they should have not debited