Got a difficult letter you want to send but you don't have the time - or maybe the words - to write it? Eleanor Rosenthal will write it for you, for a price, and she'll make it saucy or sweet depending on what you order.

One young man angered by the auto company that sold him his van, sought her help in sending a complaint to Detroit. "If I write the letter," he told Rosenthal, "it would be full of four-letter words."

"You'll want it nasty, but genteel," she advised, and began the letter:

"I want to tell you about the ride I was taken for by one of your dealers"

A Washington woman, annoyed by the clutter of election posters in her neighborhood, asked for a letter she could fire off to the new mayor. Rosenthal suggested a polite one that began:

"Dear Mayor Barry,

I am a citizen of the District of Columbia who appreciates its beauty and deplores any reaching out to make it ugly. Certainly you will agree that a political campaign message on trees or poles reaches out for votes, but results in ugliness that lingers on after the elections are over...."

The letter then politely urged the mayor to enforce the D.C. laws on campaign posters that litter.

Rosenthal's other letters for clients have ranged from one to a newspaper's circulation department complaining about late delivery to thank-you notes.

Rosenthal, who will only say she's "over 50," began the business last fall from her Chevy Chase, Md., home. It hasn't flourished as much as she had hoped - "not nearly as phenomenal" as the Chicago woman who, according to recent wire service reports, put an ad in a weekly and got orders for 48 letters in the first two weeks. But she is encouraged. Her letter-writing service, she says, is becoming known by word-of-mouth and by occasional ads in suburban weeklies.

Rosenthal decided to "capitalize" on her letter-writing experience because of her long background in public relations, mostly for non-profit organizations such as Common Cause, where she was often asked to write letters. And some things, she says, are better said in a letter than on the phone. "When it's written, it's forever."

She takes notes by hone (656-1873) and then reads back a draft.She'll also do the typing, but most people, she says, prefer to put the letter in their own handwriting and on their own stationery. The charge is a minimum of $10 a letter for the first half-hour of her time. An additional half-hour is also $10.

So far, no letter has taken more than an hour, but it might if she were called upon, say, "to resolve a 20-year estrangement between a mother and daughter."