price not the same

I was at total garage this morning at 7:30, wanting to buy a peppermint crisp chocolate, so I had R21 with me and on the shelf it reflected R23.50, so went back to the car to fetch the rest of the money, when I got to the counter it reflected R27, and I told the cashier what it says on the shelf and she then ignored me and assisted the delivery guy, and I ask her so why is it lesser on the shelves and she said to me that's how its been done, and said it with attitude and looked away and assisted the delivery guy. Pathetic service, didn't even bother to go check the shelf with me or call the supervisor, such service makes me angry cause I've just decided I'm no longer going back there