Fomiss Shell Taking advantage

Myself and my wife topped up her Audi A1 1.0 2018 TFSI which has a 42 liter tank . We got to the Fomiss Shell Garage on Xavier with just above 3 quarter tank full. The charge should have been for +/- 10 liters, however we were charged for 29.4 liters of 95 petrol. I was not paying attention ,my wife indicated that they filled a water can behind the vehicle with the excess petrol as she heard the noise of the cannister been filled . This is Theft