You can print a permit on each piece instread of using stamps. This saves you the trouble of sticking a stamp on each piece in your mailing. To use a permit imprint, all pieces in the mailing must weight the same. The permit must be legible and cannot be handwritten. The imprint must include the mail class (for example, PRSRT STD or FIRST CLASS), the statement "U.S. POSTAGE PAID" the city and state where the permit is held, and the permit number. Here are some examples of valid permit imprints

a simple way for Department of Defense installations to pay postage

a simple way for DOD installations to pay postage