Rigid car rental contracts

We took out a rental agreement that includes 100 km a day. On the 2nd morning we phoned Cape Town to try and upgrade contract with more km, but we were turned down twice. The guy assisting me became irritated and started arguing with me. We drove more then the 200 km and were charged excess km. I phoned Durban and left several messages for Essa, but he never returned my calls. Bella took my number and promised to phone me as soon as Essa was done with his phone call as I complained that he never phones me back - this never happened. Eventually I mailed Woodford and explained my situation, requesting that they \upgrade"the contract and pay back the balance of excess km charged. Bella rep**** on mail that Woodford will not be to change the agreement once it is already opened for rental. We used Woodford several times"