Charged with wrong price

I was at Benoni superspar Market yesterday on 20/04 by 17:20.I bought 2 packs of jumping jack popcorn and some stuff.I only realised when am home that they charged me R25.99 per pack while the price from the shelf is R19.99 per pack then i called back to the store.

They go back to check and they apologise said the system was wrong.They said i must keep till sleep save and come to the store for difference refund (1)I wonder how many customers charged wit that price. (2)How can i drive all the way from Katlehong to Benoni for R12?