Pick and Pay

Your some prices a really not for us who has 1 income.On the shelf there's a price and at the till there's a different price,why?You need to help us understand.15 kgs of maize meal is 139.99 on the shelf when l got to the till it was 159.99.How?l live far from the pick and pay l bought yesterday.How is 2,5 way expensive than 5kg maize meal?How?