The answer is no, they don't expire, but there’s an asterisk attached.
Stamps themselves don’t have an expiration date, so that book of Winter Olympics commemorative postage you bought back in 2010 is still technically good. In fact, you can use some pretty old stamps if you wanted to. According to the United States Postal Service website:
All postage stamps issued by the United States since 1860 are valid for postage from any point in the United States or from any other place where U.S. Mail service operates. You may use any Denomination stamp(s) to equal the total postage affixed. Total postage affixed must equal at least the postage charge for the class of the mail.
As USPS notes, you have to use stamps that are equal to the current cost of a one-ounce letter. Normally, this isn’t a problem with Forever stamps, which are always good. But if a stamp has a predetermined amount on it, you need to make sure it’s at least the current cost of mailing. As of this writing, it’s 60 cents. If your old first-class stamp is, say, 30 cents, you’ll need to double up to get your letter to its recipient.
Keep in mind that even though stamps may not have an expiration date, they can still be rejected if you apply tape over it, if it’s overly worn, or if it’s otherwise illegible.
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