Bad service rather no service at all

They have a special box in the vegetable section showing price as R39.99 there were potatoes 7kg in the box. Whe we got to the till it scans as R72.99. They refuse to give it as price advertised on box and suddenly say there is no price on those items. Then they should have given it for free as there is no price, correct? The staff at the vutchery was missing for 15 minutes when she showed up to help she was on her vell phone. Worst experience ever