If a stamped letter is found to require more postage, the amount lacking is stamped on the letter, and must be paid by the receiver.
Stamped envelopes and stamped wrappers are sold by the post office at the usual rates of postage, with the cost of the paper added.
If a stamped envelope or wrapper is spoiled, the stamp must not be cut off and used by pasting on another envelope or wrapper, for it will be treated as if no postage were paid.
Such spoiled wrappers or envelopes will be exchanged, without charge, by the postmaster, for stamps of the same value.
POSTAL CARDS
Never use a postal card to dun a debtor.
Never send a confidential message on a postal card.
Foreign postal cards, that is those bearing a foreign stamp, cannot be used in the United States.
An international postal card can be bought.
Postal cards and letters may be redirected and forwarded without extra charge, where the address of the receiver has been changed. Packages require a renewal of payment in such cases.