AFAIK a legally binding contract only happens once you actually exchange money for a product. That should be true pretty much all over the world as long as there’s actual laws/customs regulating this process.
Any prices/offers or whatever else you might see in or around a store are in no way legally binding no matter how believable the prices are.
Otherwise anyone can just run around with a 10/20/50% off sticker and force any store to sell them whatever they want for much cheaper.