Except as provided otherwise in this chapter, a joint operating agency shall purchase any item or items of materials, equipment, or supplies, the estimated cost of which is more than $30,000 exclusive of sales tax, or order work for construction of generating projects and associated facilities, the estimated cost of which is more than $150,000 exclusive of sales tax if more than a single craft or trade is involved, or $75,500 exclusive of sales tax if only a single craft or trade is involved, by contract in accordance with RCW 54.04.070 and 54.04.080, which require sealed bids for contracts.