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 fifteen thousand dollars exclusive of sales tax, or order work for construction of generating projects and associated facilities, the estimated cost of which is more than twenty-five thousand dollars exclusive of sales tax, by contract in accordance with RCW 54.04.070 and 54.04.080, which require sealed bids for contracts.