Increases in the rates of the taxes under this subchapter do not apply to building materials purchased by persons engaged in constructing, altering, repairing or improving real estate for others when the materials so purchased by those persons are affixed and made a structural part of real estate in the fulfillment of a written contract for a fixed price not subject to change or modification, or to a formal written bid that cannot be altered or withdrawn, if the contract is entered into or the bid is made before the effective date of the sales and use tax rate increase.