Inconstant Debit Order amounts for the same contract

Telkom mobile for the last 3 months have been debiting my account exorbitant amounts sighting that I am using out-of-bundle rates. I have 2 data contracts with them, at 100 bucks a month each. However as of late, amount like 800, 600, 200, have been debited off my account. Question is, When I run out of data(which happens very rarely) I buy data cash.. Now They can't even give me the answers I'm looking for. Sort out your mess - I can't wait to get over this contract nonsense. I done