Postage Due - I
By 1854, the Post Office Department required prepayment of postage.
It was the need to show evidence of prepayment of postage that eventually resulted in the invention of the adhesive postage stamp.
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.
While at various times some countries have simply adopted the expedient of returning the letter to the sender, many others have taken the approach of delivering the letter and collecting the fee from the recipient.
but this was subject to abuse by mail carriers, who might write it on themselves and pocket the difference
The problem of underpaid foreign mail was one of the issues addressed by the 1874 establishment of the Universal Postal Union. The U.P.U. 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.
Later more countries started to use handstamps to indicate the amount due.
Later the combination of handstamps with both the "T" and the amount missing came into use.
From October 1, 1907 the rules were changed.
The amount due would be charged instead of the amount missing would be indicated. The amount charged was usually double the amount that was missing.
Generally, the shortfall will be caught by a postal clerk or mail carrier and the item will be returned to the sender for payment of the missing amount.