Because according to Article 41 of the Contract Law, if there is any dispute about the understanding of standard terms, it shall be interpreted according to the usual understanding. If there are more than two interpretations of the standard terms, an interpretation that is unfavorable to the party providing the standard terms shall be made.

I think putting up posters in the shopping mall is an offer.

Because the nature of the mall itself determines that the customers it targets are not specific, and the product range is not specifically indicated in the poster of the mall, it can be inferred that all the trademarks of the mall are within the scope of buy one get one free, and the price of the products can be determined, because the mall is clearly marked and conforms to the characteristics of preferential treatment.