Upon the indorsement of any of its member banks, which shall be deemed a waiver of demand, notice, and protest by such bank as to its own indorsement exclusively, and subject to regulations and limitations to be prescribed by the Board of Governors of the Federal Reserve System, any Federal reserve bank may discount or purchase bills of exchange payable at sight or on demand which grow out of the domestic shipment or the exportation of nonperishable, readily marketable agricultural and other staples and are secured by bills of lading or other shipping documents conveying or securing title to such staples: Provided, That all such bills of exchange shall be forwarded promptly for collection, and demand for payment shall be made with reasonable promptness after the arrival of such staples at their destination: Provided further, That no such bill shall in any event be held by or for the account of a Federal reserve bank for a period in excess of ninety days. In discounting such bills Federal reserve banks may compute the interest to be deducted on the basis of the estimated life of each bill and adjust the discount after payment of such bills to conform to the actual life thereof.