I am paying for another company's courier charges

The billing system at Pietermaritzburg branch has for the second time made a mistake in billing me for someone else's parcel. Mistakes happen, no problem, I understand. But when I have to repeatedly phone and notify them of the mistake and nothing is done to rectify it, that is what i find concerning. The second error occurred early Jan 2019. I have phoned, I have phoned some more, I have e-mailed and still the problem persists. What is going on here, are you *****ing from some clients to pay other clients bills? And the problem is that all accounts are pre-paid, so the money is sitting in my account, Courierguy grabs it and it seems I have no recourse. I think it is time to look for another courier company out there.