From time to time, I have been branded an apologist for the United States Postal Service because I refuse to accept the doctrine that USPS should deliver 100 billion letters a year without any mixups whatever.

Inasmuch as even columnists make errors occasionally, I am inclined to follow the charitable philosophy that Jane Ace preached so persuasively: "Remember, you're only human once."

A recent letter-to-the-editor from Wilfred Davis referred to Bill Gold's "repeated negative reports" on the USPS. I loved it.

There is nothing quite as beneficial to a reporter who wants to establish a reputation for impartiality as being criticized by partisans on both sides of a controversial issue.

It may interest Davis to know about the first two reactions to his defense of USPS. S. Subramanian of Gaithersburg informed me that on March 4 he placed in a Bethesda mailbox a check for his mortgage payment addressed to a company in Crofton, Md. The company received his check several weeks later and slapped a $14.25 "late penalty" on him because the envelope was postmarked, "Prince Georges Md., March 18."

Charles Reynolds of Adelphi filed an even more interesting report. He has just received a letter of commendation from the United States Army. The letter is signed "Maj. Gen. Ivan L. Bennett, Chief of Chaplains." The letter bore two postmarks. The most recent was "Prince Georges Md., 6 May, 1980." The earlier postmark was, "Washington, D.C., 11 p.m., April 30, 1954."

Permit me to repeat a theme that has appeared here before: There are many explanations for long delays in the delivery of mail. The most frequent of them is that the letter was found in a mail sack that was thought to be empty.

But there is only one way to explain a service that risks embarrassment by delivering letters years after they were posted, although it could quietly burn them instead, or bury them in landfills the way the government buries office furniture when it wants to buy new things.

If USPS hid its mistakes, nobody would be any the wiser. To its credit, it does not do this. Instead, it delivers long-delayed mail and risks ridicule and criticism. Giver USPS credit for that.

The fact that USPS fouls up occasionally isn't the issue. What's pertinent is: How good is its batting average? What do we expect for 15 cents, superhuman perfection?

If you have dialed 100 billion phone numbers and never once misdialed, you are hereby granted permission to criticize USPS.

However, if you have compiled something less than a perfect record in 100 billion tries, you are permitted a choice of only two alternatives:

1. Hereafter, note USPS deficiencies with compassion, or

2. Keep putting coins into a pay telephone until you have dialed 100 billion consecutive numbers without any goofups.

If you run out of nickels and dimes before the experiment is completed and must leave the phone booth to get change, start over with Call 1. AS OTHERS SEE US

Even those who have worked for newspapers for decades are sometimes surprised at reader reaction.

Here's an example:

A few days ago, cartoonist Hank Ketcham drew a Dennis the Menace cartoon that showed Dennis and his mother driving home from the supermarket. Dennis was checking through the bags of groceries, and the lines under the cartoon informed us that he was saying, "Turnips . . . spinach . . . carrots! Boy, you're really mad at the world today, aren't cha."

This cartoon was another in Ketcham's series on the little boy's dislike for vegetables. But in drawing the figures, Hank put a shoulder strap around the mother -- quite probably because he had, on a previous occasion, drawn people in an automobile who were not wearing seat belts, and he had been criticized by safety-minded readers. So this time he put a shoulder harness around Mama, and I am getting letters asking, "Why is Dennis permitted to roam around in that car without a belt?He needs it more than the mother does. Do you realize how many children are killed or injured in auto accidents each year because they weren't strapped in?"

I'm sure Hank Ketcham will receive 100 such letters for every one I get, and I feel rather sorry for him. His intentions were good, but that counts only with God. Readers have more demanding standards. POOR MOM

Bob Orben says he's dissapointed that candy manufacturers have permitted another Mother's Day to go by without supplying us with a box of candy appropriate for the mothers of teen-agers: Chocolates with Valium Centers.