A warehouse operator must ship grain, at the request of the receipt holder who returns a receipt with orders to ship, within the time period specified by the shipping order, or within the time period specified by written agreement that clearly supersedes the shipping order. A written agreement does not supersede a shipping order if it was signed prior to receipt of the shipping order and clearly states that the written agreement shall control time of shipment while the agreement is in effect to the exclusion of any shipping orders. In the absence of a time period for shipment in the shipping order or the absence of a written agreement, grain must be shipped within a reasonable time, not to exceed 48 hours unless approved by the department.