The director of purchases and any person or officer authorized to purchase materials, supplies and services for any state agency or unified school district shall purchase, except as otherwise provided in this section, the products and services on the list certified by the director of purchases from qualified vendors, when those products are to be procured by or for the state or unified school district or when those services are to be procured by or for the state. Services offered for purchase are not required to be purchased by a unified school district. The person or officer authorized to purchase materials, supplies and services for any municipality may purchase the products and services on the list certified by the director of purchases for qualified vendors.