All dealers, handlers, or processors who purchase commercial tree fruit from a producer for sale, processing, or shipment anywhere, shall withhold the assessment due and payable the Washington tree fruit research commission by producers of such commercial tree fruit unless adequate evidence is supplied by such producer that payment of the assessment has been or will have been made directly by himself. The first handler is responsible for payment of the research assessment, but he shall charge the same against the producer, who is finally responsible for such assessment. A producer who transports his own fruit or fruit on consignment to fresh market is deemed to be a first handler.