AS A CHILD of the '60s I am willing to buy any conspiracy theory about our government. The latest one going around has to do with how the post office is handling the mail, now that it is facing its 1,987th financial crunch.

The theory is being pushed by Herman Talcott, whose book, "Today's Post Office Could Never Find Lincoln's Gettysburg Address," has been on the best-seller list for months.

Talcott told me, "Have you ever wondered why you can get a catalogue from Hammacher & Schlemmer the day after it's posted, but it takes two weeks to receive a check mailed to you from the same city?"

I admitted I had.

"Well," he told me, "the post office has installed new computers that sort out the junk mail from the letters you are really waiting for. The junk mail gets first priority and first-class mail goes out only when there is space available in the mail pouch."

"I can't believe it," I said.

"It's true," he said. "Friday I got a catalogue from Neiman Marcus in Dallas, Tex., which was mailed on Thursday, and a letter from my Aunt in Bethesda which said, "Grandpa died this morning. His last wish was to be buried in Arlington Cemetery. Could you ask President Eisenhower if he could arrange it?"

"That's strong evidence to back your theory," I admitted.

But I need more than that to make me believe the post office is not doing a good job.

"All right," Talcott said. He took out two envelopes. "I got both of these on the same day. One was from Gun and Rod magazine offering a 50 percent discount if I subscribed by April 30, 1980. The other was a 'Dear John' letter from a college coed I was going to marry after the Korean war was over.

"Do you need further proof?" Talcott said. "My American Express bill arrived at the house the same day it was sent out. A letter from my agent asking me if I was interested in writing the screen version of 'Casablanca' came in two days later."

"How does the computer distinguish between mail you don't want to get and mail you're waiting for?"

"It has a scanner which reads the addresses. All mail addressed to 'Resident' get sorted first. Then any letters that have a cellophane window are neatly placed in the same pile. Those that say 'IF YOU OPEN THIS ENVELOPE NOW YOU COULD WIN A MILLION DOLLARS' get special handling. Then the scanner picks out all the electric, gas and oil bills to make sure they're delivered on time."

"And finally it gets around to sorting the first-class mail?" I asked.

"No," he said. "Then it breaks down, and takes a week to be repaired."

"Well, no computer is perfect," I said. "I'm sure the post office will get its scanner bugs worked out in the next decade."

"Even if they did," Talcott said, "they have a fail-safe system to make sure your first-class letters don't arrive before your bills and junk mail."

"Are you sure?"

"I'm certain of it. The post office has developed a new conveyor belt to mutilate any personal letter that slips through the system. Any hand-addressed envelope with a 15-cent stamp will automatically self-destruct once it hits the belt."

I still wasn't about to buy Talcott's conspiracy theory until I got home that evening and found my wife crying. "We've been invited to the White House for a State Dinner."

"Well, why are you crying?" I asked.

"It's being given by the Lyndon Johnsons for Charles De Gaulle."

"I guess it's too late to reply," I said.

"The worst part is I got my new Lord and Taylor's spring catalogue at the same time with the perfect dress I could have bought for the dinner."