Dstv incompetence

Had dstv for many yrs, have been paying my account via debit order all this time. Started spinning their wheel won free subscription. They didn't debit me for 2 months even though debit order was jn place. Original debit order is R440, then juky they debit me R190, now this month August I'm told I owe R809. When I queried it this was their response

Unfortunately, credit was duplicated on your account in error and we have to reverse the extra credit provided, thus the balance, unfortunately, payment was not made. We have acknowledged our error but the payment is still needed on the account.~TM

How can they just debit me such a high amount without first consulting me, who gave them the rite to just change my debit order amount...why was I not consulted to make some payment arrangement for outstanding balance which they made an error of not deducting.

So disgusted with their customer service...we live in hard times and every penny counts...why must I suffer for their incompetence.

What can I do please help