Incorrect Pricing - No Refund

I purchased the Milky Bar chocolate from CHECKSAVE in Queen Street. The item was marked R6.99 on shelf. I ended up paying R9.99 at the counter. As soon as I realised I was overcharged by R3 whilst in store I approached the teller and some staff members who stated that the item had the old price and I can only exchange the item for something of that value. My argument was that I purchased the item by mistake and now I no longer require it so I have nothing to exchange it for. It's not my fault that I purchased it as how was it supposed to know the **** price is wrong? Regardless I was refused a refund and told that \this is CHECKSAVE