You don't have to be a stamp collector to enjoy and collect FIRST DAY COVERS.
ALL THE FIRST DAY COVERS THAT I SELL ARE AUTHENTIC
AND ISSUED BY THE UNITED STATES POSTAL SERVICE. THEY ARE EITHER COMMERICALLY MADE OR LIMITED EDITION ADD-ON CACHETS.
An ADD-ON cachet is:
"A cachet that has been added to a previously uncacheted fdc,
usually years after the FD."
From The Cachet Identifier of U.S. Cacheted First Day Covers
By Michael Mellone and Barry Newton
The add-on could be hand painted, hand drawn, computer printed,
rubber stamped, glued photograph to name a few methods,
and made in very limited editon, many are one-of-a-kind.