Jerry Seinfeld is set to appear on a U.S. postage stamp — and he’s not even a President. The TV series “Seinfeld” is but one of 15 stamps depicting ’90s pop culture to be issued on May 2 by the U.S. Postal Service. As part of its “Celebrate the Century” commemorative series, the post office is issuing stamps depicting such artifacts of the last decades as the movies “Titanic” and “Jurassic Park,” cell phones, the World Wide Web, the Persian Gulf War and sports utility vehicles. Selections were made by public balloting. What, no “South Park”?