Postage due use
Definitive stamps were not intended for use as postage dues, but it is not an exceedingly rare practice to use them that way. These covers are examples.
Half-cent
One cent
One cent coil
Two cent
Three cent
Three cent coil
Four cent
Five cent
Six cent
Seven cent
Eight cent
Nine cent
Ten cent
Twelve cent
Fifteen cent
Nineteen cent
Twenty cent
Thirty cent