*postal*

a. [ Cf. F. postal. ] Belonging to the post office or mail service; as, postal arrangements; postal authorities. [ 1913 Webster ]

Postal card, or
Post card, a card used for transmission of messages through the mails, at a lower rate of postage than a sealed letter; also called postcard. Such cards are sold by the government with postage already paid, or by private vendors without a postage stamp. The message is written on one side of the card, and the address on the other. --
Postal money order. See Money order, under Money. --
Postal note, an order payable to bearer, for a sum of money (in the United States less than five dollars under existing law), issued from one post office and payable at another specified office. --
Postal Union, a union for postal purposes entered into by the most important powers, or governments, which have agreed to transport mail matter through their several territories at a stipulated rate. [ 1913 Webster ]