VODACOM incorrect billing

I upgraded one of my numbers (it was over the upgrade date by more than 2 months thus, the handset was already paid off) and took out new line in the middle of August this year, Both was for exactly the same special deal on VODACOM online. Huawei P8 lite 2017 on Uchoose Flexi 120. The only difference was that I took a gold phone for the upgrade and a white phone for the new line. The sales person I spoke to assured me that I would be paying the special deal amount for both phones. When I got the activation SMS'es that same day, the amounts was different (more) from the amount that was advertised on the special. I phoned the department to find out why this happened and I was told that there were issues with the SMS system and that I should not worry about the SMS with the wrong amount, I will be paying the special price. I followed up on this once or twice more and was told the same thing. When I got my email with my invoices I was charged for the incorrect amounts. I called and mailed VODACOM about this. I was contacted by them to resolve the matter. They ended up resolving another issue I was not asking about, I asked about the incorrect amounts on my invoice again and was told that they need to listen to the recordings etc. and someone will get back to me. I have followed up again with the same result. It is now 2 months later and I got my new invoice still with the incorrect amounts, and it gets debited from my account. I feel that it is futile to contact VODACOM again, wait for how long ever to speak to the correct person, explain the whole story AGAIN and then to be told again that the matter will be looked in to. I just want to pay the correct amount on my account.