Double the difference not appropriately app****

I went to pick n pay to buy a heater, the price on display was 370. I went to pay and noticed tht i had paid 380, i then went to kiosk to complain bout the price difference , i was assisted by margarietha who went to the cashier that was in kiosk (margaret kwenaite) and asked her to give me the 10 rands i paid extra,i then told margarietha that im not supposed to be given 10 rands but 20 rands instead, both margarietha and margaret looked surprised then i told them bout the double the difference, so margarietha told me to wait as she wanted to check something, we wasted time waiting for margarietha to confirm the policy thts already displayed for everyone to see, she came back and asked margaret kwenaite to give me 20 rands instead of 10 rands. The problem i have is that both margarietha and margaret made me feel like it was wrong and unnecessary to complain bout the difference nd the double the difference policy.so it seems like customers at pnp r being ********** by cashiers not applying the policy cz i wasnt suppse to tell thm bout th policy thy shud hv knwn bout it.pick n pay please educate ur staff bout the policy nd avoid lettin custmrs being **********.practise wat u display