British Commonwealth
Postage Dues
To solve the problem of mail with insufficient postage paid, postal authorities issued Postage Due stamps. Any item underpaid by 1d would be charged double i.e. 2d, and had to be paid by the recipient. Postage Dues have always been a great source of revenue for postal authorities so mint issues are relatively easy to find. But a properly used not 'cancelled to order' can be quite rare to find.