One price on the shelf and another at the till

I bought Ariel Washing powder 2kg at Clicks today and the price displayed on the shelf was R39,99. When I arrived home something made me check my till slip. Then I saw that they charged me R65,95 instead. Tomorrow I have to go back to the store again to complain. This is not the first time this type of thing has happened to me at various Clicks branches. Somebody is dishonest and I have had enough of this now.