Bad service at Engen Glendower Service Centre in Edenvale

I’ve been filling up petrol at Engen Glendower Service Centre in Edenvale for several years and never had problem with their staff, but felt very disappointed today. After I paid him cash, he asked me to give him extra money because he have exceeded the amount that I asked him to fill, at the same time he kept looking into my car like searching for money or something. It’s not about the Extra amount that I have to pay, but the way that he kept searching into my car making me feel very uncomfortable. Seems like he is new because I have only see him twice. I think Engen need to do proper training with their staff.