In Author goes in search of 'Calvin and Hobbes': Author details mission of writing about reclusive cartoonist Bill Watterson by Maria Sciullo, Pittsburgh Post-Gazette January 20, 2010, she writes:
Characteristically, Calvin and Hobbes are making grotesque faces on the stamp. It's part of an ongoing series honoring 100 years of comic strips in America, said Terry McCaffrey, manager of stamp development for the U.S. Postal Service.
" 'Calvin and Hobbes' came to the forefront" of the citizen's stamp advisory committee, he said. "It was an extremely popular choice."
Contacting Mr. Watterson through his agent, he said, they were surprised when he turned them down.
"His concern was he didn't want any product created around it. We said, 'We can certainly take that off the table, we just want to commemorate your great work on a postage stamp.'
"And he agreed to that."
And note the 'ongoing' in the quote - yeah!