To deal with this issue, a shopkeeper has drinks for sale. He puts them in the fridge with a large sign advertising the price. A customer sees it and offers to buy.
The shopkeeper does not have to sell the drinks if he does not want to. At that stage there is no contract.
The shopkeeper is making what lawyers call an 'invitation to treat', an invitation to the customer to make him an offer.
So the customer cannot insist on buying the drinks at the advertised price. If the shopkeeper has put an unusually low price on it - deliberately or by mistake - he can refuse to do a deal with the customer.