Clayton Fritchey writes ("Tax Robbers," op-ed, Dec. 31) that the problem with the economy and the reason we can't collect enough taxes to balance the budget is the "shadow" economy. He cites the "underground economy" of illegal activities such as drug trafficking, prostitution, gambling and so on. No one disputes that. But he goes on to say that business activity that is legal generates income that individuals fail to report. He says that "up to 50 percent of all rental and royalty income is not reported, with landlords, writers and oil-well operators the principal guilty parties."

I don't know about the other so-called culprits, but I have always been paid by check for my writings. I have never been handed cash by editors or publishers, as Fritchey states--the obvious reason being that they are writing off their payments to me. And when the IRS audits writers, it usually runs into small pickings. According to a recent survey by the Communications Research Center of Syracuse University, the average free-lance net income in 1976 was $7,835 and in 1980 it was $9,700. Some big underground economy that is.

My husband is also a big "tax robber," because he's an outside salesman for life insurance. He has never been paid commissions in cash. His checks come from a computer and copies of his W2 forms are sent to the IRS.

Neither of us has a salary, we get no cost- of-living increase, we get no paid holidays and we don't make any money unless we hustle and use our wits. Granted, there are not so many self-employed people left in this country, and we're certainly not organized into viable voting blocs. But we, as a true minority, resent attacks on this last vestige of individual enterprise.

Now, as for auditing, this is the unkindest cut of all. It begins with a notice in the mail, followed by weeks of scurrying through old files collecting proof of deductions. Two minds are tied up in anxieties while unknown numbers of well-paid personnel build a case and wait for the kill. Unlike a murderer, the taxpayer is guilty until proven innocent and time spent during the agonizing proving process is just so much tough luck down the tubes.

According to Fritchey, there are 18,500 IRS employees but there ought to be more. More? The plush offices in modern high rises that resemble a Chrysler or Lockheed suite are just waiting for the briefcase toting saviors of our tax system. Bring them on. Hundreds of them, thousands of them, millions and billions and trillions of them.

On the other hand, we might consider abolishing the IRS altogther. Everybody, including corporations, would be taxed at maybe 3 percent across the board. The fairness of this proposition will satisfy the poor, the poor poor, the working poor and the working poor poor, and no administration need fear that discrimination is rampant.

As for the IRS personnel, they can be lined up shoulder to shoulder at areas where illegal drug trafficking is going on. They could join the Coast Guard or be used in Justice Department investigations. They could masquerade as almost anybody and do us a world of good in ferreting out the crooks in our midst.