Page "Postage due" Paragraph 10

While, technically, there is no reason for postage dues to reach private hands unused, postal administrations have sold them to collectors, and the postage dues of many countries exist in large numbers, often unused and of low value.

Conversely, surviving examples of valid postally used postage due stamps tied together by a dated cancellation or other postal markings with a postage stamp on cover are less common and are sought after by a philatelists.