It shall be the duty of the purchasing agent aforesaid to contract for all supplies, merchandise, printing and articles of every description needed for the maintenance and operation of each county office, department and institution, except those supplies that are of a strictly perishable character, basing his contract or contracts upon estimates to be furnished him by the county officers and heads of county departments and institutions. Such estimates shall be furnished by the first day of July of each year, for an entire year; and all such contracts shall be made after full notice by advertisement of not less than 4 weeks in at least 4 of the leading newspapers of the state to be selected by said agent. Such advertisements shall call for sealed bids or proposals to furnish the aggregate of the desired article and supplies as estimated for by such office, department or institution, naming the articles and supplies and the quantities and character required, and all such bids or proposals shall be for the entire period of 1 year; such supplies, articles, and merchandise to be delivered at such times and in such quantities as said agent may from time to time designate, and should the supplies, or any portion thereof, as contracted for, be not sufficient for the year for which the contract or contracts shall be made, then the contractor or contractors shall be required to furnish such additional supplies at the prices named for similar articles under contract or contracts: Provided, That should said purchasing agent at any time discover that he could purchase the same supplies for less money for any 1 year by buying the same for less time than 1 year, he shall have the authority to make such purchase for a shorter length of time, but not less than 3 months, it being the purpose of this act to authorize and require said purchasing agent to make such contracts upon such terms as will secure the best and cheapest rates for the county in the purchase of articles and supplies of necessity for said offices, departments and institutions, and to that end he shall reserve the right to reject any and all bids, or to accept any bid in part or reject it in part; and if none of the bids and proposals are deemed advantageous and satisfactory, he may buy in the open market until a proper and satisfactory bid is offered. The period for which such bids or proposals are invited shall be clearly stated in said advertisements, as well as the terms and conditions contemplated by the provisions of this act. When the same article is estimated for by 2 or more offices, departments or institutions, but of different brands or grades, the purchasing agent may determine which of the brands or grades shall be purchased so as to produce uniformity in use by all the offices, departments and institutions: Provided, That other things being equal, supplies offered by bidders who have an established local business in the county shall have preference.