Overcharged

I ordered a meal online for collection yesterday and the total on the online system and my calculation added up to 145 however when I got to the store I was made to pay R153. I questioned why and they didn't give me a reason. I got home and double checked everything and I was certainly overcharged. I reached out on their twitter and didn't get a respond. Imagine. The store didn't care, twitter page could give less, so how many more people are being overcharged....?