Incorrect billing

So yesterday I went to engen filling station in moutsia mall(Iam not sure with the spelling) next to siyabuswa. I asked them to fill my tank I was on 240 and some odds km still on my tank(3 bars). I didn't look if the machine was refreshed before he assisted me or what. What didn't make sense is the fact that I paid R1150. 35, where else I know normally when my tank is empty I would fill my tank with around R1200 to R1300. I saw the car in front of me paid with cash and didn't stay for long and he then the petrol attended didn't go to pay before assisting me.. I then thought to myself"could it be that my bill was added from the previous car's bill? " I asked the guy if he refreshed he said yes,i wanted proof of that I got non. He said to me there is no proof that could be provided to me. Why was no supervisor called to assist in this matter? It's the first time in my life having to complain about fuel bill. I always pour full tank in my car and it can't be now all of a sudden m bring delusional. I demand proof from engen that will assure me that there was no foul play on the above. And if there was, why? I know mistakes happen but if there was a mistake done, I could have been given an ear and probably got a solution but not be dismissed like thst.