Incorrect credit card deductions

We rented a vehicle from OR Tambo for 2 weeks. While the vehicle rental was great, the whole experience has been marred as I've been over charged. A refund voucher that I app**** on my initial booking has been disregarded and the full invoice cost was deducted off my credit card. I have been struggling for the last 3 weeks to resolve this with customer service to no avail. I've requested that customer service put me in touch with the financial manager but this was ignored. Every email I send to customer service, they repeatedly send me my R8.4k invoice to which I repeatedly illustrate that my invoice has been settled in full via my credit card, proving that my R5.2k voucher was not app****. I've been going in circles and feel like customer service do not understand the issue. I've run out of options and patience, so I see now other way to resolve this privately than to reach out on this platform to please request to discuss this with someone in finance who understands debits and credits ?