OVERCHARGE

I purchased Pet Club specials on Sheba cat food at Equestria branch in July. The till receipt did not deduct the discount advertised. I queried the cashier and the manager, but with the shrug of the shoulders was referred to Head Office. I emailed customer care including a copy of the receipt and a Screenshot of the special, but was eventually told the matter will be referred back to the branch. Of course that was the end of the matter. How do I get the R95 overcharged back if no one at Pick n Pay will take responsibility?