First Rental Overcharge

We recently rented a vehicle from First Rental, when my husband rode off after signing for the vehicle he noted the fuel gauge was just under full. I advised First Rental by email, never received any feedback. When we arranged for the vehicle to be collected, same place we took rental from. We filled the vehicle with petrol and the gauge showed full when handing back the vehicle. We were however charged R110.20 plus an admin fee for fuel. We emailed asking for tracking so could see if the Driver went anywhere after we handed over, we have been ignored. We cannot be liable for fuel after we handed the vehicle back. A couple of months ago we hired from Avis, exact same process followed re filling up the vehicle and we had no issues. It is not a lot of money but if First Rental does this a number of customers they are pocketing a nice extra sum at the Customers cost. We will never use First Rental again, firstly no feedback when we advised the vehicle was not full and then no feedback when we requested info on the vehicle after we handed back to First Rental representative.