The bad news about credit cards is that merchants do boost prices slightly to cover the costs of processing card transactions, penalizing shoppers who pay cash.

The good news is that credit cards generally don't seem to make shoppers more vulnerable to impulse buying, as many critics of the credit card economy claim. Nor do they turn otherwise frugal consumers into big spenders.

These and other findings come from a detailed study of credit cards by the Federal Reserve Board requested by Congress in 1981 and released to the public yesterday.

Congress, worried that credit card users are pushing up prices for all consumers, has taken several steps over the last nine years to encourage merchants to give discounts of up to 5 percent for customers willing to pay in cash.

Legislators believed that the higher costs of credit cards end up in higher prices for goods and services that are paid for by all customers.

Except for gasoline stations, 25 percent of which offer such discounts, few retailers give price breaks to customers who pay in cash, the study reported. The reason may be that credit-card expenses have a relatively small impact on a retailer's costs.

Many gasoline purchasers--the only group of customers who regularly come into contact with cash discounts--pass up the opportunity to take the cash discount because they prefer the convenience of charging their fuel.

The study said that costs of a sale by credit card are higher than those for cash or checks by about 2 to 3 percent of the amount of the sale. But because only 15 percent to 30 percent of total sales are accounted for by credit cards, the merchant is likely to raise prices to cover credit-related costs by "less than 1 percent," the Federal Reserve concluded.

About 70 percent of all U.S. families own some type of credit card--gasoline, bank, travel and entertainment, and department store. More than 60 percent of those families actually use one or more of those cards. Department store charge cards are most often used, followed by bank cards such as Visa and Mastercard.

The Federal Reserve study said that the higher costs associated with the use of credit cards approximate the fees credit card companies charge retailers for each charge slip. Those fees range from 1 to 5 percent of the sale, with the average around 3 percent, the Fed said.

Some retailers--oil companies and departments mainly--maintain their own credit card operations. But the interest received from customers usually does not cover the full cost of the retailer-owned credit operation. As a result, whether retailers use their own cards or accept third-party cards like Visa or American Express, credit-card sales are costlier than cash sales.

But even though most families have at least one credit card, the vast majority of sales still are cash.

As a result, the study reported, "Total sales might be expected to incorporate a premium for credit costs (uncovered by credit revenues) ranging from less than 0.5 percent to perhaps 0.25 percent, some part of which would still be born by credit card users in proportion to their 15 to 30 percent share of total sales."