When the estimated total cost of construction, erection, demolition, alteration or repair of a public improvement exceeds the competitive bid threshold in section 26.3, or as established in section 314.1B, the municipality shall advertise for bids on the proposed improvement by two publications in a newspaper published in the county in which the work is to be done. The first advertisement for bids shall be not less than fifteen days prior to the date set for receiving bids. The municipality shall let the work to the lowest responsible bidder submitting a sealed proposal. However, if in the judgment of the municipality bids received are not acceptable, all bids may be rejected and new bids requested. A bid shall be accompanied, in a separate envelope, by a deposit of money or a certified check or credit union certified share draft in an amount to be named in the advertisement for bids as security that the bidder will enter into a contract for the doing of the work. The municipality shall fix the bid security in an amount equal to at least five percent, but not more than ten percent of the estimated total cost of the work. The checks, share drafts or deposits of money of the unsuccessful bidders shall be returned as soon as the successful bidder is determined, and the check, share draft or deposit of money of the successful bidder shall be returned upon execution of the contract documents.