There is hereby levied, and the director or his or her designee shall collect, upon each and every affected unit of any agricultural commodity specified in any marketing agreement or order an annual assessment which shall be paid by the producer thereof upon each and every such affected unit stored in frozen condition or sold or marketed or delivered for sale or marketed by him or her, and which shall be paid by the handler thereof upon each and every such unit purchased or received for sale, processing or distribution, or stored in frozen condition, by him or her: PROVIDED, That such assessment shall be paid by producers only, if only producers are regulated by such agreement or order, and by handlers only, if only handlers are so regulated, and by both producers and handlers if both are so regulated. Such assessments shall be expressed as a stated amount of money per unit or as a percentage of the receipt price at the first point of sale. The total amount of such annual assessment to be paid by all producers of such commodity, or by all handlers of such commodity shall not exceed four percent of the total market value of all affected units stored in frozen condition or sold or marketed or delivered for sale or marketing by all producers of such units during the year to which the assessment applies.