That might be what they say, but this says different
The first phrase is invitation to treat this basically refers to the action of a seller, merchant or trader advertising or displaying their goods or services that are on offer. When the consumer offers to purchase the goods or services, legally the seller is not obliged to accept their offer. As aforementioned, the contract is only made and begins once the seller has accepted the offer of purchase. This also applies when the roles are reversed and the seller makes an offer to the buyer in this case, the contract begins once the buyer accepts the offer.
The second phrase refers to something known as consideration within the contract. Essentially, consideration refers to the agreement and made by the consumer in the contract, which usually translates as an obligation to pay the seller the agreed sum of money. A contract will not exist if the seller has offered their goods or services to the buyer free of charge