Pathetic service with a manager that can't communicate well with customers

I got bad service yesterday at Game Vredenburg, I went to buy some Groceries , so I bought Oats it was a 1kg with free 100 grams for 29.99, when I get to the till they charged me 36.99 for it ,I asked the cashier she said she doesn't know why there's a price difference and called someone else that called another who told me to call Oats if I had a problem , that really upset me because I was buying at Game not oats and that oats was 1kg with 100g for free I asked what that free 100g meant he said I must call Oats and ask there couldn't I see that the sizes are different? I said I see that but it is written with big letters that the 100g is free so why are you charging it, do he took it to another person who told us the same " the sizes are different" that's 1.1kg can't you see and they all laughed, then I asked them then what's does this 100g free mean and then they told me they didn't know, the worse part is there was no price for 1.1kg there, there was 1kg price only and for that manager to tell me to call Oats and ask why a 29.99 Oats is 36.99 was the worst thing I've ever heard from a shop, the other even said they don't put prices themselves so I must call Oats, I asked him didn't Oats give us that 100g for free he said I should ask them or complain at Oats, I said no I'll complain at your customer services he said I'm welcome to do that an left me there, tried to call their customer services which made me hold until I was out of of my 60 rands airtime