Billing issues on the system and they backdate all the invoices?

I called in to WebAfrica to cancel my internet service with them effective end October 2017 on the 01 October 2017 they debited an irregular amount from my account so a called in to find out, that for the past 3 months they did not bill for the 200GB data package on the account but they did bill and collect for the Fibre line therefor on the 01 October 2017 they collected the last 3 months of data bills. I asked them how is it possible that they can bill and collect the actual Fibre line but not the data package and the consultant rep**** it was a system/agent error... So for a system or agent issue you backdate and collect as you want? I would not have minded if they called and said we apologize for the inconvenience sir but we did not bill your data for 3 months so we will have to collect X amount from your account I would have said sure, but no warning no nothing just "that is how it is sir". Thanks for bad service... luckily I cancelled WebAfrica Fibre. Will never use WebAfrica again for internet service provider.