Make it make sense R400 lasted 1 day after driving 70km

I want to file a complaint with BP Seebran Motors in Durban. I acknowledge that I made the mistake of failing to verify that the person filling up had truly filled up R400 when I filled up my tank with R400 on Saturday, October 12, in preparation for work as usual. The question of why she was swiping my debit card while she was clearly still filling up my tank struck me. My grievance is that I usually fill up between R400 and R500 for work, and it lasts me the entire week. Today, though, I have to fill up because the petrol only lasted for one day. After what happened, I know that something does not add up. Is this ***** garage still part of BP. I live less 3min away from the garage and to work and back is 70Km I did not use the aircon even if I did it would not have consumed that much fuel and Noone else drove my car.