Billing amount not the same as debit amount

My invoice for September was R990.99 but I got debited R1138.71 the first of October<br> <br> After calling the call centre, apparently this was for a 1 gig data bundle purchased in August (which doesn't appear on the August or September invoices). The person at the call centre said that the discrepancy was due to Vodacom changing their billing system, however I find it completely unacceptable that a multimillion rand company cannot get its invoicing right or even warn customers of the possibility beforehand.