In any case where under the provisions of a contract it is provided that solid fuel be sold at cost by an employer-producer to his employees for their own use and consumption, the solid fuel may be sold by cubic contents instead of weight, but no solid fuel so sold shall be transported over the highways of this Commonwealth from the place of production to the residence of the employee unless the operator of the vehicle possesses a certificate of origin. The certificates of origin shall contain such information as may be prescribed by the department and shall be signed by the producer or the producer's agent, and a copy of each certificate shall be kept at the place of production for at least two years.