Why lie to customers?

I went to checkers in Eastgate mall, I saw a promotion saying when u buy 500g provitas for R38.99 u get 175g Marmite spread free, to start with , I don't get 175g Marmite, I asked, the manager tells me they made a mistake with sizes, they don't sell 175g Marmite @ all, I was told to take 125g since it's the 1 for R24.99 & apologised for da mistake, fine, I get to da tills, the provitas scans R29.99 not R38.99 the lady had a nerve to manually add R9 to the price, I mean whose fault is that? Checkers didn't check the real price before advertising their so called "freebie", I complained to customer service & the ladies told me I should take it up with head office since they don't deal with pricing, they just sell, I mean what the hell? Why do they hv customer service in da first place if they can't assist customers? So now I had to pay R9 for da missing 50g of my Marmite? What a Mickey mouse service? I'm not the one who made mistakes with pricing, in fact they should have given me 30% discount for dat missing 50g of Marmite, I am so disappointed on this poorest service of them all, gaa!