Section 180D(2)(i) in The Indian Post Office Rules, 1933
(i)Field Postal Order presented within four months from the last day of the month of issue are encashable without second commission. If a field Postal Order is not presented for payment within four months from the last day of the month of issue, a second commission equal to the first commission shall be charged, which shall be paid in postage stamps affixed to the back of the order.