overcharge on electricity

on every occation the past couple of years when buying electricity through FNB, i was charged the correct rate. however on 1 September 2017 i was overcharged. i contacted them and they came with the following excuse and i think that they are sucking reasons out of their thumb and i need this to be sorted, as i've been buyng electricity on the same meter number for 7 years now, and this is the first time that this has happened:

"Good day We refer to your complaint reference number ********** related to FNB Service Providers. When purchasing Prepaid Electricity various deductions may occur on the amount purchased for. A debt amount is an amount owing to a Municipality or Electricity Provider and not FNB. This may have been outstanding on a Post-Paid Electricity Bill prior to the residence being converted to a Prepaid Electricity meter system. These debt amounts can be deducted at once to settle the outstanding amount or in installments as decided by or agreed with the Electricity Provider. The Service fee displayed to you is a service fee charged by the Municipality or Electricity Provider and not FNB. This service fee is charged for services rendered to the Prepaid Electricity meter or the residence. Both the Debt Amount and Service Fee can be queried with your Municipality or Electricity Provider. It is now a requirement for the VAT portion of the prepaid electricity amount purchased to also be displayed on the Prepaid Electricity confirmation. Kind Regards FNB Tier3 ********** This email is subject to a disclaimer. Visit the FNB website and view the email disclaimer by clicking the “About FNB + Legal” and “Legal Matters” links. If you are unable to access our website, please contact us to send you a copy of the email disclaimer.""