Overbilled due to negligence from consultant

I have 3 routers with Telkom which I asked to be capped so I can be charged R99/pm for each. They have been deducting their normal instalment until today, I was billed R531.08. When I went to their store in Umlazi from which I got the router I was told that apparently one router was not capped. When I asked how is that possible because I specifically asked for them to be capped and the lady who assisted me was right there, I was told that “Maybe she forgot to do it, sorry”. I don’t understand how I have to pay for your employees negligence, there should be way in which such errors should be written off on your side and not have to be paid for by the client as if we are at fault.