"Doonesbury" has done it again. This time, the joke is on the U.S. Postal Service.

To rip the coming five-cent postage hike, cartoonist Garry Trudeau designed a sheet of 18 ready-to-use "U.S. Protest Stamps" -- each featuring Zonker, the comic strip's star troublemaker.

Several thousand Americans have clipped the full-color, life-size "-5" stamps from the Sunday comics and stuck them on letters in place of the legal kind, postal officials estimate.

Each bogus stamp that slips past detection machines wears on the Postal Service's sense of humor.

"Either Garry Trudeau didn't have a better idea or he was somehow irritated that the price of stamps is going up," said spokesman Lou Eberhardt. "That's a pretty easy target: postage rates."

In the May 20 strip, which appeared in more than 900 newspapers, Zonker invites readers to "affix these dandy -5 protest stamps to all your mail" to challenge the February 1991 hike from a quarter to 30 cents.

Half the stamps say "Mad as heck." The rest say "Ticked off."

"So go ahead, slap on a U.S. Protest Stamp!" Zonker chuckles in one panel. "They're attractive, convenient and possibly illegal!"

"No, they're definitely illegal," Eberhardt snapped. The fine: $300.

One piece of mail, postmarked Phoenix, was delivered in Broward County, Fla., late last week. Somehow, the cancellation machine didn't get the joke.

"It never should have gotten through," Eberhardt said.

No one is more surprised at the response than Lee Salem, Trudeau's editor at Universal Press Syndicate.

"It's only just a comic strip," Salem said.