Wikipedia上给出了一个非常清晰的解释：A postal order or postal note is a type of money order 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 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.

大概意思为：邮政汇票或邮政票据是一种通常用于通过邮件汇款的汇票。 它在邮局购买，并在另一个邮局支付给指定的收件人。 服务费（称为手续费）由购买者支付。 在美国，这被称为邮政汇票。 邮政汇票不是法定货币，而是一种本票，类似于支票。

据此Postal order可译为：邮政汇票；汇票

例：If we request payment of the administration fee we will not reply until after we have received your postal order or your cheque has cleared.

 如果我们要求您支付手续费，我们将在您的汇 票 或支 票到帐后给您回复。

In this context cash means coins and notes in any currency, postal orders, travellers’’ cheques, bankers’ drafts, and such other kinds of monetary instruments  as the Attorney General may specify.

在此背景下，现金 指的是任何币种的硬币和纸币、 邮政汇 款、 旅行支票、银 行汇票以及司法部长可 规定的其他货币票据。

此外，在英国还有一种特殊的邮政汇票叫Promotional postal order。具体英文释义为：A promotional postal order (PPO) is a special type of postal order that is issued in the United Kingdom by various companies in conjunction with the Royal Mail to promote their goods and services. These have been issued at various times since the 1970s.

附：List of companies known to have issued PPOs

Cinzano

Smash

Nescafé

Typhoo

Reader's Digest

Maxwell House

Bounce (fabric softener)

Swan

Brooke Bond Teas

Persil

Kleenex

Cannon

Post Marketing Surveillance Unit

Berkeley

Philips

Boehringer

Tango

General Guarantee

Fife Council (These are the only Scots postal orders.)

Uplands

Argos