Great Britain
Postage Dues
Mail with insufficient postage applied presented a problem for postal authorities, and Postage Due stamps were created to solve it. If an item was underpaid by 1d, it would be charged at 'double the deficiency', i.e. 2d, to be paid by the recipient. Postal authorities have nearly always viewed selling mint Postage Dues as a means of raising revenue, so often mint stamps are relatively easy to obtain, but properly used (not 'cancelled to order') can be a different matter and covers showing correct non-philatelically inspired usage are often rarities.