Paying more than what was agreed

I don't know if this is my unlucky year with Vodacom or what but it seems like you can not get my debit order right and it is affecting me financially. With my last upgrade you guys deducted too much from me and after complaining on HelloPeter you helped and figured out it was a systems issue on your side. After receiving the call to advise it was a mistake on your side it was confirmed what my debit orders will be going forward but I was proven wrong again as for the last couple of months my debitorders was more than what was agreed. Vodacom I am at a loss for words as to what I should do .