How were postage-due stamps used?

Postage-due stamps are specialized adhesive stamps applied by postal authorities to mail that has been sent with insufficient postage.1

In the context of the postal system, these stamps serve as a mechanism to collect the remaining fees required for delivery from the recipient. While early postal history was characterized by a system where the recipient typically paid for postage upon delivery, the 19th-century reforms led by Rowland Hill shifted the standard toward prepayment by the sender. Postage-due stamps were subsequently introduced to manage instances where this prepayment requirement was not fully met.231