Mo. Rev. Stat. § 144.730: Advertising that vendor absorbs tax, a misdemeanor.

No vendor shall advertise or hold out or state to the public or to any customer, directly or indirectly, that the tax or any part thereof imposed by sections 144.600 to 144.745, and required to be collected by him, will be assumed or absorbed by him, or that it will not be added to the selling price of the property sold, or if added, that it or any part thereof will be refunded. Any person violating any of the provisions of this section is guilty of a misdemeanor.