Postage due

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 of underpaid foreign mail was one of the issues addressed by the 1874 establishment of the Universal Postal Union.

arrived at the decision that mail with insufficient postage should be marked with a "T" and from April 1, 1879 the amount missing would also be indicated in black.

[1] The problem was solved by France in 1859, with the issuance of official postage due stamps,[2] affixed at the delivery office before being taken out to the recipients.

Postage due stamps (or "labels", to clarify that they have no value of their own) are not always affixed to individual letters; in the case of business mail, the total due might be summed, and the appropriate stamps added to the top letter in a bundle, or to a bundle's wrapper.