The commission on a postal order is known as poundage. Poundage is the fee charged by the post office for providing a postal order service. It is usually a percentage of the total amount of the order and is added to the cost of the order. This fee is charged to cover the cost of processing the order and to provide revenue for the post office. The amount of poundage charged may vary depending on the amount of the order and the location where the order is being sent.