Is it possible to reuse stamps?
A used stamp cannot ever be reused. If the stamp was already used to send mail through the USPS®, it cannot be used to mail a different mailpiece even if the stamp was not canceled.
Stamps that have not yet been used, but have been attached to an envelope may still be used. However, you may not cut an unused stamp from an envelope and use tape to apply to a new envelope. The tape will prevent the stamp from being canceled.
A returned domestic or international shortpaid mailpiece can have the necessary additional postage affixed to the original piece and does not have to be placed in a new envelope or wrapper.
A returned undeliverable as addressed or refused by the addressee mailpiece may not be remailed unless it is placed in a new envelope or wrapper with the correct address and new postage.