Nkandla Engen Garage with poor services on item brought from the inside shop

During holiday i was at home in kzn Nkandla and decided to go and fill up gas in my car. During time of filling the gas in the car i sent my younger brother to buy some Simba chips unfortunately he came back with wrong flavor so when i went with him and receipt they refused to change it for us. I was so disappointed same time angry because i used to trust Engen gas station a lot as i always fill my car there. The staff there have attitudes towards customers. please resolve this issue with Nkandla Engen Garage