Sec. 13-18   Purchasing and contractual procedure
   (a)   The City must provide, by ordinance based upon a national standard, for a purchasing procedure to be followed in purchasing city supplies, materials, equipment, contractual services, or other personal property. Before making any purchase or contract to purchase, competitive bids or proposals must be obtained, except when the purchasing official for the city is exempted from the bidding requirements by the purchasing ordinance because of value or when the city council determines that no advantage to the City would result from competitive bidding or a request for proposal.
   (b)   Purchases must be made from the lowest responsible bidder or party making a proposal which meets specifications, unless the city council determines that the public interest would be better served by accepting a higher bid or proposal or by rejection of all submitted bids or proposals. At any time that the bid of the lowest responsible bidder is not accepted, the city council must specify the reason in its resolution concerning the purchase.
   (c)   The City must provide in the ordinance the definition of “lowest responsible bidder,” the dollar limit within which the purchasing official may make purchases without the necessity of obtaining competitive bids, and the dollar limit within which purchases may be made without city council approval.
   (d)   The purchasing official must provide the city council with all certifications required by law prior to city council action on any contract. The city manager must, before the final payment for any work for which a bond has been furnished, report to the city council whether the work has, in the city manager’s judgment, been done in all respects in accordance with the specifications. The report does not waive any rights the City may have to pursue any action for non-compliance.
   (e)   No contract shall be split into multiple contracts in order to avoid any provisions of the bid procedure.
   (f)   All contracts require the signature of the mayor, city manager, and city clerk.
   (g)   When it becomes necessary or prudent during any work or improvement project done under contract to make substantial alterations or modifications in the contract which exceed an amount set by resolution of the city council, the alterations or modifications must be made only upon resolution of the city council. The city council must by resolution determine what is “substantial” and may do so as a general policy or on a contract-by-contract basis. No order may be effective until the price to be paid for the material, or work, or both, under the altered or modified contract has been agreed upon in writing and signed by the contractor and the city clerk, upon authority of the city council, and a copy of the modification documents are filed in the office of the city clerk.