Stamp on a 2 dollar bill....

how did it come to have stamp and postmark?
In 1976, the Treasury Department reintroduced the $2 bill as a cost-saving measure.[20] As part of the United States Bicentennial celebration, the note was redesigned and issued as a Federal Reserve Note. The obverse featured the same portrait of Jefferson, a green instead of red seal and serial numbers, and an engraved rendition of John Trumbull's The Declaration of Independence on the reverse. First day issues of the new bicentennial $2 bills could be taken to a post office and stamped with the date "APR 13 1976".