Postnet sending to PTA instead of NELSPRUIT and we must pay to fix their problem

Postnet Rynfield Terrace - we send a parcel from postnet to postnet for Nelspruit, THEY deliver it to Pretoria, when we follow up their answer is: POSTNET has done it's job by delivering the parcel and we need to pay ANOTHER R100 to get it to Nelspruit, was this not YOUR mistake, WHY SHOULD WE PAY??????