Incorrectly charged for goods

I ordered a grande americano coffee from Happy.Me in Menlyn. On their customer display, the price reflects as R20.50. I was charged R24.50 for the coffee though. When explaining to the server that according to sections 23(6) and (7) of the CPA, I am only liable to pay the lowest price, she explained that the 'system' wasn't allowing her to give it to me at the correct price displayed. Not wanting to take it out on her (it wasn't her fault), I explained that I would lodge a formal complaint. Unfortunately, Happy.Me doesn't respond to emails. So, my next step is to resort to the National Consumer Tribunal. It really sends a bad vibe to consumers when we are charged incorrectly for goods, and there are \no managers available"in store to sort out the problem for you - almost seems purposeful of them. """