Not less than once each year, the commission shall by order fix reasonable storage rates, and no warehouse licensee shall charge a lesser or greater rate, except upon application to and a hearing before the commission. If, after a hearing, the commission finds that a lesser or greater rate should be ordered, it shall enter a supplemental order fixing a different rate for such applicant. Such charges shall be full compensation for the receiving, handling, storing, delivering, and insuring of grain. No discrimination shall be made between different customers by any state-licensed grain warehouse either in facilities, charges, or handling of any grain, except that members of a cooperative may be given preference in storage facilities in warehouses of the cooperative. The rates charged to any governmental agency shall be exempt from commission regulation.