Overcharged and minimum help!

I've purchased the special on sparletta sodas on Sunday. The deal was 3 for R30, however after I received my order I noticed that I was charged an extra R10 on each 3 for R30 deal. I've purchased 3 of these deals so that means I was charged R30 extra. I've tried emailing sixty60 to rectify this mistake however all the assistant did was tell me there was no error. How is this possible? I have screenshots of everything to papertrail this. I'm requesting further assistance from checkers to view this again. Preferably someone who is literate in maths.