Over Charging Clients at sertain Branches

My mother often use Postnet to send a package to her grandkids in Herm****, so on 06 November 2017 she send a package and paid a lot more than she was supposed to.

I contacted the Branch manager in Witbank Highveld Mall to explain this to me, she said they are allowed to make their own pricing. I contacted Head Office Via E-mail, I received a mail back from David Liebenberg saying that the calculations I have is correct I was over charged the manager in Witbank will contact me, so I haven't heard any thing back I send follow up mails : 05/12/17 19/01/2018 & 07/02/2018 I also **** a call and spoke to David he said the manager will phone me, so no one did, and I want my money back that I was over charged as I constantly use this way to post and I feel this is unfair to me as a client that actually supports this company monthly.

tracking no:PPA ********** 74691

I know normally it is R99 for up to 5Kg and anything over is R20 So my package (PPA ********** 74691) was 14Kg – Volumetric

5Kg R99 9Kg X R20 =R180 +R99 =R279

But I paid 5Kg R99 9Kg XR45 =R405 + R99 = R504 ?