Section 20. If the board of aldermen of a city or the selectmen of a town named in the agreement of association, after such notice, exhibition of the map and the hearing, agree with the directors as to the said route or as to any route of the railroad in said city or town, they shall in such agreement fix the route, and sign and give to the directors a certificate setting it forth. The board of aldermen of a city or the selectmen of a town will also set a fee for such a certificate.