Postage due stamps are stamps which are affixed to mail by postal employees to indicate that postage is due. Most nations today use a rubber stamp which reads “postage due” with a line to fill in the amount of postage due, rather than using specialty stamps. When someone receives mail which has been marked “postage due,” they must pay the postage due before they can collect the mail. This is often done by putting a notice in someone's post box to indicate that mail with postage due is waiting at the post office.
In the early days of organized mails, when letters were sent without sufficient postage, they were often returned to the sender, with the sender being expected to remedy the situation before mailing it again. Insufficient postage could happen because someone didn't know how much to pay, or because someone was hoping to sneak a letter past the postal officials; either way, returning it to sender was the approach taken to deal with it.
Eventually, post offices developed an alternative. Instead of returning to sender, they would deliver the letter, but note how much postage was due, with the recipient being expected to pay on the other end. However, some people feared that unscrupulous mail carriers were taking advantage of this practice by marking mail “postage due” and pocketing the funds. As a result, the postage due stamp was introduced, with the stamps being pasted onto the letter or bundled onto a stack of mail, and the consumer paying the postage indicated on the stamps.
France appears to have been the first nation to introduce the postage due stamp, and it caught on quickly. By being held at the post office, the stamps could be secured, so that people trying to generate some extra funds from their mail routes would not be able to pocket postage due. These stamps were also sometimes used to charge people for magazine subscriptions and certain other mail order items.
Some stamp collectors are interested in postage due stamps, although they are generally less valuable than regular postage stamps. They also tend to be less visually interesting. Guides to value are available for people who wish to collect them, and photographs of some notable postage due stamps can also be seen in guidebooks. Stamp collectors are also often happy to show off their collections to people who are curious, so those with philatelists in their acquaintance may ask to see some real-life examples.