insulted by petrol attendant

I went to the engen garage on DF Malan Dr in Pretoria on the 29 May 2015 to fill up my tank which was already 90 percent full because I was travelling. I queried the amount being charged as the bill was rising to 300 and going for a tank that is almost full when it takes less than R600 for my cars full tank. I the petrol attendant Jeofrey verbally attacked me with vulgar language and saying that in this place the customer is not always right. his colleague then came to find out what was happening without being given a chance to explain they both charged at me telling me that I am wrong and think that because I am all that I can come and undermine them in their work place they both refused to call the manager for me and never apologised for the way I was treated I sent several mails directly to engen with no response. I still don't know why I paid almost 300 for less than a quarter of a tank and I still do not have an apology.