Good way to check: Get a tank that holds exactly 12.5 gallons of gas. Then put 12.5 gallons (according to the pump) into that tank. If the tank is not full, it is cheating you (if it fills short of 12.5 gallons, all the better because that is in your favor).
If the error is more than a few milliliters, you should document it. Then, report it first to the station, and then to the authorities if nothing is done about it. I believe there are laws that ban charging for phantom products, as this is fraud. (However, an honest price of $6 a gallon, if displayed honestly, should not be illegal if a station sees fit to charge that much).