Should the Commission determine that it is necessary or desirable that a given project be performed in phases, it will not be necessary to negotiate the total contract price in advance. The Commission shall then contract with the firm it determines is best qualified to perform the whole project at reasonable cost. Such a contract shall contain provisions specifying that the Commission may utilize the firm for other phases and that the firm will accept a fair and reasonable price for subsequent phases to be later negotiated, mutually agreed upon and reflected in a subsequent written instrument. The procedure with regard to estimates and negotiation shall otherwise be applicable.