Upon the request of any department head or upon the purchasing agent’s own request, the purchasing agent has authority to procure goods, services, equipment, or other personal property with a cost of less than twenty-five thousand dollars (small purchases) by issuing a purchasing order to the applicable vendor. The purchasing agent shall engage in reasonable diligence to assure that small purchases are obtained upon the best possible terms, considering price, quality, reliability, availability, and performance of the vendor.