The fees charged on postal order by the post office is

When a customer buys a postal order from the post office, the post office charges a small fee for issuing it. This fee is called poundage. It is calculated as a percentage or fixed charge based on the value of the postal order being bought, and it is the post office's way of earning income for providing the money-transfer service.

The other terms belong to different contexts. Premium is the payment made for an insurance policy. Interest is the charge paid for borrowing money or the return earned on savings and investments. Brokerage is the commission paid to a broker for arranging a transaction, such as buying or selling shares. None of these describe the specific fee charged on a postal order.

Examination reminder: associate poundage specifically with postal orders and money orders; it is a term unique to this service and is frequently tested on its own.