In the era of stampless letters, it was generally accepted that the recipient would pay the postage on a letter at the time that it was delivered.
People would normally only write to someone if they knew that the recipient would be able and willing to pay the postage to receive the letter.
The provisions of most postal authorities permitted short-paid letters to be delivered, with the balance due collected from the recipient.
Postage due is the term used for mail sent with insufficient postage. While the problem of what to do about letters not paying the full correct fee had existed since the creation of regular postal systems, it was greatly heightened by the advent of postage stamps, since customers were now making their own decisions about the right amount to pay, without the assistance of a presumably-knowledgeable postal clerk.
The problem was solved by France in 1859, with the issuance of official postage due stamps, affixed at the delivery office before being taken out to the recipients. Many other countries followed suit. (Occasionally, regular postage stamps have been used to fulfill a postage due function.
Since postage due stamps are almost always used only within a single country, they are usually quite simple in design, mostly consisting of a large numeral, and an inscription saying “postage due”, “porto”, etc; often there is no country name.
Read also