Commemorative stamps are bought by collectors as well as people intending to use them to mail something. Most collectors buy stamps soon after they're issued, the report said, leading the inspector general to recommend limiting initial production of stamps and printing more if demand warrants.
The Simpson stamps, sold in 2009 and 2010, came in five designs featuring Homer, his wife, Marge, and children Bart, Lisa and baby Maggie. The stamps sold for 44 cents, 1 cent less than it costs now to mail a letter.
The inspector general criticized the process the service uses to decide how many stamps to produce, saying it's unscientific and too much of a judgment call. The July 23 audit was reported earlier by Linn's Stamp News, a weekly magazine for collectors.
"This process depends on manual procedures and the experience of one individual, which increases the risk for costly miscalculations," the report said. "Further, such errors may be detected if an independent review and assessment of production estimates were performed."
The Postal Service, in a response to the report, said it addressed the problem by creating the "forever" stamp, which can be used to mail a letter any time in the future. The Simpsons products were printed when most stamp values were fixed, meaning they could no longer be used by themselves to mail a letter after postal rates increased.
"The forever stamp has gone a long way in preventing overproduction," said Janet Sorensen, director of marketing and service in the IG's office and leader of the audit team that produced the report. "They need to get a better process for projecting the need, and they are implementing that type of process."
While the Simpsons stamp was the most overproduced during 2009 and 2010, the service also produced more stamps that it sold in those years featuring the lunar new year, civil rights movement figures, Zion National Park, Supreme Court justices, historic U.S. flags, film director Oscar Micheaux and a Christmas stamp showing an angel with a lute.