What’s the cost of a stamp?

Priceless, it seems, to the estate of Roy Orbison, which has ramped up an effort to get the late rock crooner honored by the U.S. Postal Service.

There’s a push on several fronts.

For starters, an Orbison cassette — remember those? — was included in bags given to 10,000 attendees at March’s SXSW music fest, along with a written plea to help the stamp project.

Sony Legacy has also issued a two-CD hits compilation, plus the video of “Black & White Night,” a star-studded 1987 Orbison concert. XM Radio declared April “Roy Orbison Month” in honor of what would have been his 70th birthday. And they donated enough items to the Rock and Roll Hall of Fame to get a special Orbison exhibit.

All for a stamp.

The estate’s online petition has more than 15,000 signatures, including those of U2’s Bono and Pamela Anderson.

While the USPS has honored a fair number of film stars and songwriters, rock and roll is not common on stamps — even though the Elvis Presley stamp is the biggest seller in Post Office history.