Poor service delivery

Pick N Pay at Vincent Park East London had special of 2kg Tastic Rice (26.11.21) - you buy 3 (6kg ) for R60 , when you get at the shop you get 1kg of Tastic Rice which you have to take 2 to make 2kgs , So I wanted 18 kg of the special, I took 16kg that is in 1kgs and 1- 2kg to make my 18 kg

I only notice the slip when getting at home that the other 6kg charged normal , I went back to the shop hey say there is nothing that the can do I should have notice that while I was still at the shop