Longman Dictionary of Contemporary English
postal order
noun
EXAMPLES FROM CORPUS
▪ Coins Any uncrossed postal order which does not state to whom it is to be paid.
▪ If you paid by postal order, take the counterfoils to the post office for a refund.
▪ Please return it at once, or when sending the postal order.
▪ Send the coupon, and cheque or postal order if applicable to,.
▪ You should print your name and address on the back of the cheque or postal order.
Wiktionary
postal order
n. (context British Ireland English) A money order issued by, and redeemable at, a post office.
WordNet
postal order
n. a written order for the payment of a sum to a named individual; obtainable and payable at a post office [syn: money order]
Wikipedia
Postal order
A postal order is a financial instrument usually intended for sending money through the mail. It is purchased at a post office and is payable at another post office to the named recipient. A small fee for the service, known as poundage, is paid by the purchaser. In the United States, this is known as a postal money order. Postal orders are not legal tender, but a type of promissory note, similar to a cheque.