“Shipper” means any person who performs any of the functions of shipping strawberries within California. When the shipper is a corporation, all of the directors and officers of the corporation in their capacity as individuals shall be included, and any liability for failure to collect or make payment of assessments for which a corporate shipper may be subject pursuant to this chapter shall include identical liability upon each individual director or officer of the corporation. It does not, however, include a retailer, except a retailer who purchases or acquires from, or ships on behalf of, any producer, strawberries that were not previously subject to assessment by the commission.