18 U.S.C. § 1721

Whoever, being a Postal Service officer or employee, knowingly and willfully: uses or disposes of postage stamps, stamped envelopes, or postal cards entrusted to his care or custody in the payment of debts, or in the purchase of merchandise or other salable articles, or pledges or hypothecates the same or sells or disposes of them except for cash; or sells or disposes of postage stamps or postal cards for any larger or less sum than the values indicated on their faces; or sells or disposes of stamped envelopes for a larger or less sum than is charged therefor by the Postal Service for like quantities; or sells or disposes of postage stamps, stamped envelopes, or postal cards at any point or place outside of the delivery of the office where such officer or employee is employed; or for the purpose of increasing the emoluments, or compensation of any such officer or employee, inflates or induces the inflation of the receipts of any post office or any station or branch thereof; or sells or disposes of postage stamps, stamped envelopes, or postal cards, otherwise than as provided by law or the regulations of the Postal Service; shall be fined under this title or imprisoned not more than one year, or both.