The authority shall proceed with the acquisition, construction, installation and equipping of the judicial facilities as soon as may be practicable following the sale of the authority's bonds. Such acquisition, construction, installation and equipping shall be done by the authority under the supervision of the Building Commission upon the award of a contract or contracts for each part of the work to the lowest responsible bidder after advertisement for and public opening of sealed bids; provided, that for the purpose of determining the lowest responsible bidder, the invitation for bids and the bidding documents shall be so arranged that alternates from the base bid shall constitute cumulative deductions from the base bid in the event such alternates should be selected. All such contracts shall be lump sum contracts and shall be awarded and executed by the authority to the respective lowest responsible bidders following determination by the Building Commission of the lowest responsible bidder. Payments made under any such contracts shall be only upon the contractor's written, verified statements when approved by the Building Commission or its agent, in such amounts as the Building Commission may approve as having been then earned under such contracts. The Building Commission shall be reimbursed for all reasonable direct costs incurred by it in connection with such acquisition, construction, installation and equipping, including expenses and fees for the preparation of plans, specifications and contract documents and supervision and inspection of the work. Such plans and specifications may be revised and extras may be added to the contracts only when approved by the Building Commission and only to the extent that funds are available therefor.