Lyndee Nelms thought her billfold would be safe in her desk. But when she looked for it, it was gone.

The cash involved was the least of Lyndee's worries. The billfold had contained the usual assortment of credit cards and other papers. If you need a reminder of how many of these things we carry, take inventory of your own wallet and you'll know what gloomy thoughts coursed through Lyndee's mind.

But just as she was about to begin the task of notifying a dozen organizations of the need to cancel the old cards and issue new ones, the billfold was returned to her by the Swift Couriers of the United States Postal Service. After the thief had helped himself to the cash, he had dropped the billfold into a mailbox. USPS immediately forwared the billfold to Lyndee.

She's grateful to the Swift Couriers for taking so much trouble with an item found "loose in the mails", and she's also glad the thief jettisoned the wallet in a mailbox. Had he chucked it into a trash can, there is little chance she'd have seen it again. THE LARGE ECONOMY SIZE

Mrs. B. A. Dickerson of Kensington was buying sugar in a Giant Food supermarket. She was astonished to find that the 10-pound bags were marked $3.09 and the 5-pound bags were 99 cents.

"I would really like to hear Giant explain this one," she said when she reported the incident to me.

I put the question to a Giant spokesman. "Yes," he confirmed, "that's the way the packages are supposed to be marked."

Why? The answer will be obvious to merchandisers, especially people in the food business, but perhaps not so obvious to others.

"No-frills" supermarkets have come into this area recently. They carry a limited number of items, offer fewer services, but pass the savings along in the form of lower prices -- especially the prices on advertised specials.

Concerned about this new form of competition, the old-line chains have tried to meet no-frills prices. When a no-frills store cuts sugar from 30.9 cents a pound to 99 cents for 5-pound bags, Giant and some others meet the price but leave their 10-pound bags at the regular price.

A spokesman at Giant told me that much agonizing went into the pricing decision. "We have four responsibilities," he said. "The first is to serve society well; we must treat our customers fairly. The second is to our employees and their families; and we must operate at a profit to pay them well and give them an opportunity to advance. Our third responsibility is to be fair to our suppliers. And our final responsibility is to the investors who have entrusted their money to us and are entitled to a fair return on it.

"We can't cut every price in the store and remain economically viable, but we do feel obliged to meet reduced prices offered by others, even by stores that give the public a sharply curtailed selection of goods and fewer services.

"Having said all this, we still are not happy about leaving 10-pound bags on the shelf at a higher unit price than is available in 5-pound bags. We don't want to take advantage of the shopper who doesn't have time for unit price comparisons. Our solution to the problem has therefore been to begin posting 'Best Buy' signs on the shelves. We hope those signs will alert shoppers to the fact that, temporarily at least, a larger size does not carry a lower unit price than a smaller size."

So a word to the wise is in order: It pays to take the time to compare unit prices -- and to look for those "Best Buy" signs. 'TIS THE SEASON

Tomorrow is Dec. 1, and our annual Children's Hospital fund drive officially begins on that date. This year we have some good news for you.

For more than 30 years, I have given up the usual District Line fare during December and January to devote this space to fund raising. Henceforth, however, readers will have both a District Line column and a fund drive.

Scott Chase is going to conduct the Children's Hospital campaign in space immediately below this column while The District Line continues to present the same kind of material it busies itself with during the other 10 months of the year.

To make the new system work better, please do this: If your letter deals only with the hospital's fund drive, please address it to Scott Chase. If it deals only with District Line fare, send it to me as usual. If it deals with both, address it to either of us but put separate messages on separate pieces of paper. Scott will hand me your District Line items, I will hand him the checks and the accompanying messeges that explain how the money was raised. Our address is: The Washington Post, 1150 15th St. NW, Washington, D.C. 20071. Thank you.