Notwithstanding the provisions of chapter 42.24 RCW or any other existing statute, school districts and other public agencies including but not limited to state agencies and municipal corporations which are expressly or by necessary implication authorized to subscribe to magazines or other periodical publications or books or to purchase postage or publications from the United States government or any other publisher may make payment of the costs of such purchases in a manner as consistent as possible and practicable with normal and usual business methods, and in the case of subscriptions, for periods not in excess of three years.