From the Leaky Cauldron:
"Floo Powder" Shuts Down Post Office
A post office in Buckner, Missouri was temporarily shut down when a Harry Potter themed birthday card was discovered to have white powder inside.
"A worker at the Buckner post office spotted white powder on an envelope. The office was sealed off and hazmat crews were called in to check out the situation. ... The letter turned out to be a birthday card a 16-year-old fan was sending to Harry Potter in England. The teen apparently sprinkled the card with some "floo powder," which turned out to be harmless talcum powder.
Thanks reader Sue!
From the Daily Prophet:
Obliviators were called to the scene of a Buckner, Missouri "post office" (Muggle center for mail distribution) when Floo powder was found on and around a birthday card.
Fortunately, the quick-acting American Magic Council officials were able to replace the Floo powder with harmless talcum powder before the arrival of Muggle hazardous-material handlers, who were called in under the suspicion that the powder was diseased.
The incident was blamed on a careless secondary-school student.
"The sixteen-year-old perpetrator has been reprimanded," said an official, regarding the indiscretion. "But, geez. Use an /owl/ next time."
The card was addressed to Harry Potter, The Boy Who Lived and well-known local crackpot.