With respect to marketing agreements, the Commission shall, after publication of the final decision, invite all producers affected thereby to agree to the marketing agreement or amendment set out in such decision. The marketing agreements or amendments shall be binding only upon persons who have agreed in writing thereto and whose written agreement has been filed with the Commission; provided, that the Commission shall enforce a marketing agreement or amendment only when it shall be found in addition to the other findings specified in this chapter that such marketing agreement or any amendment thereto has been assented to by a sufficient number of signatories who produce a sufficient volume of the commodity affected and sufficient moneys from assessments levied to defray the necessary expenses of issuance and enforcement is provided. The marketing agreement shall be deemed to be issued and in effect when the Commission has notified all persons who have assented thereto.