Price tag does not match till slip

went into the festival mall musica store to purchase a few dvds and 2 hdmi cables, after paying i got home and checked the till slip and thew price on the product, i was charged R30 more than the price on the item, this isnt the first time ive noticed this happening. the last time i purchased a dvd it was marked 2 for R100 yet i was charged R129 for the dvd. this is horrifying as how many other people are experiencing this, i sometimes do not even check my receipt.