There is hereby levied, and the Commission shall collect, upon each affected unit of any agricultural commodity specified in any marketing agreement or order an annual assessment which shall be paid by the producer upon each unit sold or marketed by him. Such assessments shall be expressed as a stated amount of money per unit. The total amount of such annual assessment to be paid by all producers of such commodity shall not exceed five percent of the total market value of all affected units sold or marketed or delivered for sale or marketing by all producers of such units during the year to which the assessment applies.