The department shall redeem unused or destroyed stamps at the price paid by the buyer, provided acceptable proof of such destruction is provided the department. It is presumed that the stamps presented for redemption were the last stamps bought in the month in which the sale of the stamps was made. If the month in which the sale was made is unknown, the amount to be paid by the department upon redemption shall be computed as if the stamps presented for redemption were the last stamps bought in the average monthly number of stamps bought during the preceding calendar year.