Petrol refund

Hi, on the 12th May I hired a car 'Datsun Go' for a day but I was given a Renault KWID. I needed it to buy groceries in Randburg then drop it off at home in Soweto then back to Randburg. Always I return their cars with a full tank...So before drop off, I went to Shell garage to top up petrol for R74.50. The garage is 2min away from their offices. Now I have been charged R142.15 for petrol and they're ignoring my refund emails.