Русско-англо-русский юридический словарь

money order is an official piece of paper with an amount of money written on it that you can send or give to someone, who can then exchange it for the same amount of money (Cambridge Dictionary)

They then had to send a money order to China to pay for the file to be translated. (BBC News)

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. (Wikipedia)