Ever get a letter whose stamp hasn't been cancelled? If you're like me, you leap at such good fortune because it will be history if you don't.

"Ah, ha!" you chortle, as you gaze at the stamp in the upper right hand corner of an envelope and the cancellation mark about three inches away. "Twenty cents saved is twenty cents earned."

Out come the scissors, then the tape, to attach your "found money" to a new envelope. And off it goes.

But all too often, back it comes. Reason: it is illegal to use postage that can't be imprinted by a cancelling machine. (It's also illegal to re-use an uncancelled stamp by gluing it onto an envelope, but postal people concede that you'll probably get away with it.) But covering the face of the stamp with tape is out in any case.

But what do you do when the glue-less back is the Postal Service's fault? If you're W.G. Williams of Northwest, you find yourself in a pretty exotic Catch-22.

"Recently," writes Williams, "I bought some stamps that were somewhat lacking in the glue department . . . . I used one of these glue-less stamps to mail a letter, holding it onto the envelope with tape.

"Well over a month later, I received an 'official business' envelope from the Postal Service with 70 cents postage due. It was from a dead-letter office in New York returning the envelope with the taped-on stamp with the warning that it was illegal to reuse stamps.

"Although I was angered by the whole incident, I smile as I mention that the postage-due envelope was delivered with my regular mail and nothing was ever collected. Had I paid to get that letter back, then I would really have been angry."

Your comments, Tom Chadwick, consumer advocate for the Postal Service?

First of all, says Chadwick, Williams should have been able to exchange his glue-less wonders for glue-ful ones at any post office. "It is our policy," Chadwick says. Should Williams "or any other customers encounter a similar problem in the future, we request they ask for the postmaster or supervisor."

Secondly, Williams did not beat the Postal Service out of 70 cents. He stiffed his letter carrier.

"When a carrier leaves a postage-due item without first collecting the amount due, he does this trusting the customer will reimburse him. He must pay the charges on all unreturned postage-due mail at the end of the work day," Chadwick says.

It would be understandable if Williams had given up on the mail entirely. Instead, he says he has taken out membership in the don't-get-mad-get-even club.

"Now I just buy one cent stamps and use a lot of them," he writes. "I wonder if they count them as they are being cancelled . . . ."