All contracts shall be executed in the name of the county, or in the name of a township in a county with a township form of government, by the head of the department or officer concerned, except contracts for the purchase of supplies, materials, equipment or services other than personal made by the officer in charge of purchasing in any county or township having the officer. No contract or order imposing any financial obligation on the county or township is binding on the county or township unless it is in writing and unless there is a balance otherwise unencumbered to the credit of the appropriation to which it is to be charged and a cash balance otherwise unencumbered in the treasury to the credit of the fund from which payment is to be made, each sufficient to meet the obligation incurred and unless the contract or order bears the certification of the accounting officer so stating; except that in case of any contract for public works or buildings to be paid for from bond funds or from taxes levied for the purpose it is sufficient for the accounting officer to certify that the bonds or taxes have been authorized by vote of the people and that there is a sufficient unencumbered amount of the bonds yet to be sold or of the taxes levied and yet to be collected to meet the obligation in case there is not a sufficient unencumbered cash balance in the treasury. All contracts and purchases shall be let to the lowest and best bidder after due opportunity for competition, including advertising the proposed letting in a newspaper in the county or township with a circulation of at least five hundred copies per issue, if there is one, except that the advertising is not required in case of contracts or purchases involving an expenditure of less than twelve thousand dollars. It is not necessary to obtain bids on any purchase in the amount of twelve thousand dollars or less made from any one person, firm or corporation during any period of ninety days. All bids for any contract or purchase may be rejected and new bids advertised for. Contracts which provide that the person contracting with the county or township shall, during the term of the contract, furnish to the county or township at the price therein specified the supplies, materials, equipment or services other than personal therein described, in the quantities required, and from time to time as ordered by the officer in charge of purchasing during the term of the contract, need not bear the certification of the accounting officer, as herein provided; but all orders for supplies, materials, equipment or services other than personal shall bear the certification. In case of such contract, no financial obligation accrues against the county or township until the supplies, materials, equipment or services other than personal are so ordered and the certificate furnished.