The wonderful world of greenbacks is sometimes not so wonderful. Three cases in point:

Alexandria: Marilyn Lasser often uses the Xerox machine at the Burke Branch of the Alexandria Public Library. The charge is 10 cents a copy.

The other day, she stopped by the post office on North Pickett Street. There, she noticed, the Xerox machine costs 20 cents a copy.

"Is the federal government taking advantage of us, or is the city of Alexandria just being extra generous?" Marilyn wonders.

Neither, Marilyn. The real answer is that Alexandria's prices are low for a good reason, and the post office's are high for a bad reason.

"We keep our prices low because we encourage people to copy a page they are interested in" rather than stealing a book, or ripping out a page, said Marjorie Tallichet, deputy director of the Alexandria library. "We try to charge enough to cover the cost of the machine and the supplies."

Perfectly reasonable. But the post office charges twice as much because it is trying "not to be in competition with other vendors in the area," said George Conrad, public affairs officer for the D.C.P.O.

In other words, the Alexandria post office won't drop its Xerox prices to a dime "because it would undercut other vendors," George says.

But if the P.O. charged a dime, it would be equalling other vendors, not undercutting them. Since neither the post office nor the library is trying to make a profit from Xeroxing, wouldn't that be a painless favor?

Rockville: Stuart Krasner of Potomac ambled into the Montgomery Donut shop on Gude Drive, sat down at the counter and ordered his usual doughnut and coffee. As he waited, he noticed a sign on the wall that said, "Warm-up free." A confirmed caffeineist, Stuart was all smiles.

They turned to frowns a few minutes later when, halfway through his first cup of java, Stuart asked a waitress for more.

She charged him for it.

"Huh?" said Stuart, gesturing toward the sign on the wall. "I gave you a refill, not a warm-up," said the waitress.

At which inch does a warm-up become a refill? "To fill up half a cup of coffee is a warm-up," said Dan Troop, Montgomery Donut's general manager. A refill "is when the cup is empty." Dan promises he will straighten out the waitress who erred.

Good news. But Dan clearly has a public relations problem. Solution: Offer free "replacement" doughnuts to anyone who hasn't finished his (and invite me the day that policy begins!).

Alexandria again: Michelle S. Park writes that she has "just received my '85 proof set from the U.S. Mint. Can you guess who the shipper was? UPS! The post office used to do it, and I think the change tells a lot."

What kind of loyalty to Uncle Sam is this, Mintists? "We are supposed to compare costs and be economical in our operations," said a Mint spokesman. "We did a cost study and there was significant savings by using UPS."

When birds of its feather are deserting the postal service for greener (and cheaper) pastures, the national postal system is in sad shape, indeed.