The Post Office is generally not considered a federal agency to be trifled with. But Chicago artists Michael Thompson and Michael Hernandez de Luna just couldn’t resist, after reading about Doonesbury readers who had been trying to mail letters with fake stamps published in the famous comic strip attached, and frequently succeeding. Thompson began cranking out his own satirical stamps a decade ago, and his works have included such classics as a May Day stamp with a picture of an airline crash, and a portrait of Abraham Lincoln with a gun visible behind him. But the game turned serious two years ago, when Hernandez de Luna tried to use a stamp emblazoned with a skull and crossbones and a single word: “anthrax.”