Sure seems that way. The current postcard rate within the U.S. is 35 cents. Yet this picture postcard from my wife’s friend in Boston, from the Museum of Fine Arts, is clearly postmarked in January 2016 with a 21ยข stamp. (I’ve removed the address, but, trust me, it went through the mail.)
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Lloyd: There are many mail discounts for both non-profit organizations and for how the mail is addressed to move it through automated sorting. There is also “false franking” which subtracts the amount of the appropriate rate paid by the stamp and then the additional money is paid directly to the post office. Most likely, this is the correct denomination stamp, given the one or both of the above. –David
The card does not appear to be part of a qualified bulk mailing. If it is, the rules haven’t been followed. First of all, the class of mail is not indicated anywhere on the card. That’s required. Second, the 21 cent Lincoln stamp could probably be used for a first class mass mailing, but the stamp would have to be tied by a mailers postmark permit, not an ordinary machine cancel. The Lincoln stamp could not be used in a mass mailing under the presorted standard or non-profit rate categories. This card is more than likely just another piece of underpaid mail. – Richard L. Beecher
Richard: Agreed. I went back and looked more carefully at the card – then wrote the second post a few minutes after I wrote the first one. –David
Lloyd: On the other hand, since automation doesn’t check the denominations of stamps, it could have just slipped through. –David