All contracts in excess of twenty thousand dollars ($20,000) entered into by the district for the hiring or purchase of equipment, supplies, or materials, are of no effect unless the district shall advertise pursuant to Section 6066 of the Government Code for bids for such contracts in at least one newspaper and not more than three newspapers designated by the general manager, which are published in any one county or portion of a county within the district, and unless the district shall award the contract to the lowest responsible bidder.