A semipostal is a postage stamp for which a greater price is paid than the going postal rate, with the difference going to a charitable cause.

In the United States, semipostals have existed since 2000, when they were authorized by the Semipostal Authorization Act. The Act stipulates that "semipostal stamps should benefit only Congressionally authorized programs within U.S. Government executive agencies," as well as other criteria to ensure that only worthy causes benefit from the stamps' revenue. The differential must be at least 15 per cent of the First Class rate, and the total price must be divisible by five.

Semipostals usually bear two values (the postal rate and the purchase price) separated by a "+", or sometimes just the "+". The differential between these two values is donated to a charity, less the post office's "reasonable costs."

To date, three semipostals have been issued by the United States Postal Service. They are:

  • A 45-cent stamp (8-cent differential) released in 2002 featuring the famous photograph by Thomas E. Franklin of three firefighters raising the Stars and Stripes over the ruins of the World Trade Center, with the words "HEROES USA" in the upper-right corner, "2001" in the lower-left corner, and "FIRST-CLASS +" in the upper-left corner. The extra 8 cents is donated to the Federal Emergency Management Agency (FEMA), which will use the money to aid the families of emergency relief workers killed or disabled on September 11, 2001. The stamp was unveiled on March 11, 2002, the six-month anniversary of the September attacks.
  • A 40-cent stamp released on July 29, 1998 (revalued to 45 cents on March 21, 2002; currently 8-cent differential) with an image of a woman with arm raised and the words "FUND THE FIGHT. FIND A CURE." in the center, the words "BREAST CANCER" at the top, and "USA FIRST CLASS" along the bottom. The stamp was designed by Ethel Kessler and drawn by Whitney Sherman. 70 per cent of the stamp's proceeds will be donated to the National Institutes of Health (NIH), and the remainder to the Medical Research Program of the Department of Defense. As of May 17, 2002, the stamp has raised $26.3 million for research.
  • A 45-cent stamp released in November of 2003 with a child's drawing of a sad girl, one who (presumably) has been traumatized by domestic violence. The top edge of the stamp reads "FIRST CLASS + USA," and the lower-left hand corner reads "STOP FAMILY VIOLENCE." The image of the stamp is a photograph by Philip Channing. Proceeds from sales of the stamp are donated to the Department of Health and Human Services, as dictated by the Stamp Out Domestic Violence Act of 2001.

source: http://www.usps.com.