'Postage due' stamps were authorized by an act of Congress, approved March 3, 1879, and effective July 1, 1879. By law, postage due stamps were to be affixed by clerks to any piece of mailable matter to denote the amount collected from the addressee because of insufficient prepayment of postage.
Although the Post Office Department required postage to be prepaid beginning in 1855, there were many instances when full postage was not required and postage due payment had to be collected in cash from the addressees. These instances included insufficiently prepaid letters, advertised letters, and unpaid ship letters and steamboat letters.
The reason for the appearance of postage due stamps may be summed up in one word: accountability. The 1880 Report of the Postmaster General noted that the former system of collecting postage due had one great weakness: “In securing the full returns of [the postage due collected in cash] the department was entirely dependent on the fidelity of the postmasters.” Postage due stamps, affixed to underpaid mail as receipts for the underpayment collected, solved that problem by requiring postmasters to account for cash receipts that would balance any postage due stamps no longer in stock, in the same way that they had to account for regular postage stamp sales.
The last postage due stamps were printed in November 1985. With the advent of postage meters, the scrapping of the last Cottrell press in November 1985 (the last press capable of printing the current postage due stamps as designed), and finally new regulations requiring full prepayment of postage in all cases, postage due stamps became anachronistic, and their use ceased.