"Dear Friend," begins a three-page letter, "Have you heard of the 'miracle' grapefruit? I am going to send you a package of them so you can enjoy them during these cold winter months."

The pitch seems very down home, but in reality it's as sophisticated as a country music album produced in Nashville. Frank Lewis, the king of postal citrus, is trying to hook you on his "Royal Ruby Red Grape-fruit." The fruit, carefully wrapped, is delivered to your door. You pay only after you have received it. No pressure. Restrained hype.

One note he doesn't play, however, is economy. The mail-order food business usually deals with the Rolls Royces of whatever specialty is involved, be it grape-fruit, steaks or nuts. Only quality, or a supposition of quality, can justify steaks at$15 a pound or Lewis' grapefruit at nearly $1 each.

In a recent issue of Gourmet magazine, readers found ads for frozen steaks; canned Mexican-style sauce; wild mushrooms from India; snails; pecan pie; and baby veal. The previous month's offerings included maple syrup, cheddar cheese and creole spices.

It's a tricky business. The supermarket executive can count on a broad cross-section to patronize his stores, but how to price an item - usually just a matter of pennies - is of great concern. The mail-order food specialist's worry is finding and keeping customers. It costs a lot to locate them and make the first sale, then it takes care and customer service to keep them.

Price, evidently, has little to do with it. Those who buy are affluent, urban-or suburban-dwelling people with upscale life styles. They may subscribe to Fred Thomas' frozen Omaha steaks, or a shipment of pears or preserves from "Harry and David" in Oregon as well as royal ruby reds. Or they may be companies looking for a showy gift for employes or customers.

Frank Lewis, who "did business with a little over 100,000 customers last year," deals mainly with individuals. He is slender, direct and modest, a role for the actor James Stewart in his middle years and, like most other mail-order food concerns, his is a family business. "We go to great lengths to make ourselves seem like a small company," he said. But he does have a toll-free telephone number and has used a computer for more than 10 years. When he talks about customers, he talks in zip codes.

Others focus on holiday gift packs, as did Lewis's father when he began selling grapefruits from the garage behind the family's home in the 1930s. In the 1950s Frank chose to go to the University of California at Berkeley specificially to study direct mail marketing ("I went through school knowing what I wanted"). It paid off when he broke new ground in the grapefruit industry by "selling direct to the consumer for his own personal use" throughout the six-month harvest season. Even today he has virtually no competition.

One reason is that the product he sells is unusual and of very high quality. In his solicitation letters, Lewis tells how the "miracle fruit" - large, naturally sweet and with pink-tinged flesh - was found by accident. Nature, not science, had created a new strain in this border valley less than 100 miles from where the Rio Grande River reaches the Gulf of Mexico. It has been grown elsewhere since but not so successfully. Only four out of 100 grapefruit fit Lewis' self-imposed specifications for "royal" ruby reds. They are, in fact, the largest and the best of all grapefruit.

Four years ago, after a rainy growing reason that left an unusual number of fat fruit on the trees, Lewis made grapefruit juice and tested it on his customers. That "worked out wonderful for everybody," and the result allows the company to sell year-round. He also has begun marketing grapefruit sections. "It all boils down to this," he said. "A tree grows many sizes and types of grapefruits and I've got to find a home for it all. Also, I had to get into a longer cycle. There's a limit to how much business you can do during the first two weeks of December."

Selling by mail has provided the volume he needed. "With direct mail marketing," he said enthusiastically, "you can do the arithmetic and really know where you are going. If we buy a list from Omaha [Steaks], we will test 5,000 of their names, then more, then go full out. If it's a bomber, we get out early. All our orders have a source number and mail is the source of more than 95 percent."

Dick Hamilton, who oversees the mail operation, explained, "There's a big difference between mail and non-mail shoppers. We look for somebody who has bought by mail previously, somebody who has spent over $20 with another company."

This is done by approaching one of about 20 organized list brokers. These organizations have stables of mailing lists, which they rent on a one-time basis for a fixed fee per 1,000 names. The Lewis organization "likes" food lists, but has subscribed to those for a "gamut of products."

"The economics of prospecting aren't good," Lewis said. "I shoot for break-even the first year and lose on some. It's when a person repeats an order and comes on our list that we begin to make a profit.Our best market is the East Coast, the high-density areas between Boston and Washington. We don't cut much ice in upstate lowa."

Other methods of selling include magazine and newspaper advertising and solicitation by telephone, but Lewis hasn't tried television yet. Telephone selling ("very low key") is directed to former customers and Lewis also uses the telephone to deal with 80 to 90 percent of the inquiries and complaints he receives. "It's good customer relations," he said. "You just wrap it up on the telephone, make them happy and move on."

Lewis must be doing something to make them happy. Sales have moved "nicely upward" in recent years. (So have prices; a box of 16 to 20 ruby reds will cost $16.95 next fall, a dollar more than last season.) His reorder rate year-to-year is close to 78 percent. The average customer will subscribe to four shipments, then drop off but return at holiday time or in response to a new mailing or telephone solicitation.

Grapefruit growers in Texas receive 4 cents a pound for their fruit. Oranges sell for more, but it takes more effort and expense to raise and pick a pound of the smaller oranges. The grapefruit harvest begins in October, but Lewis said he delays picking to allow the fruit to become sufficiently large and sweet to meet his standards.

Lewis owns no groves himself, but has had "long, long-term relationships with 20 to 30 growers." That means "we don't have to ship fruit that doesn't suit our standards. It it doesn't meet our specifications, we don't take it. But because we are so quality-conscious, we have to pay a premium price." Each member of the team of harvesters he employs for six months a year is equipped with a ring. Unless a grapefruit is larger than the ring's four-inch diameter, it is left on the tree to mature further.

In spring, the grapefruit could pass for limes. Small and green, they grow on incredible gnarled trees with bark like elephant skin and vivid, dark green foliage that resembles large bay leaves. They are susceptible to insects and fungus, so frequent spraying is necessary and frost or a hurricane can would a grove badly. The trees "are never really clean," Lewis explained. "The new crop blooms in late February or early March, but the previous crop is being picked until May."

Subscribers' orders are shipped by truck to 50 post offices across the country, then delivered by mail. Lewis, who says he has "a love affair" with the post office, says few boxes go more than 100 or 150 miles from a drop point. New Yorkers, he claims, receive their fruit within four or five days.

Although Royal Ruby Reds account for only 4 percent of the grapefruit Lewis processes, they provide more than 50 percent of his gross receipts.

Some of the remaining fruit is sold to supermarkets in the Northwest. Another, less conventional, source he has mined in the last half-dozen years is selling the smaller and middle-size grapefruit to fund-raising organizations. Lewis provides minimum orders of 500 boxes plus promotional material. The group pays $3.95 a box (off 18 to 24) and resells them for $5.95.

As for selling other products, "we've been out and back on that," Lewis said. "Our gift package catalog used to have 36 items. Now it has 24. We tried dried fruit. We sold fancy wrought-iron baskets. They didn't do well. Our customers are looking to buy grapefruit. In the last few years we've backed off. We specialize in one product and stick to it." CAPTION: Picture 1, no caption, By Joel Richardson - The Washington Post; Picture 2, Frank Lewis, by Glenn G. Hampton for The Washington Post