Any producer, or producer who is also a vintner, who contracts or otherwise arranges for the processing of his or her winegrapes and retains ownership of the processed product during the marketing season shall pay the producer assessment and the vintner assessment. The “weighted average producer returns per ton” by district as published in the most recent Final Grape Crush Report issued annually for the current marketing season by the department shall be used as the basis for determining product gross dollar value and the assessment.