Just what the average American wallet needs most -- another plastic money card -- is about to make its debut. Not another credit card or even a debit card, this one is a national cash discount card which will entitle a shopper paying in cash or by check to an average rebate of about 5 percent from participating stores across the country.

Credit -- if that's the word -- for the cash card belongs to John M. Wilson, a Missouri lawyer, who dreamed up the idea back in 1969 somewhere between the pickles and the frozen food counter. Why should supermarket shoppers go to the trouble of pasting trading stamps in books, waiting until they have enough full ones and exchanging them for something they might not want anyway? How much easier for the customer and cheaper for the retailer to give discounts for cash payments, not only for food but for other purchases as well, he thought.

The concept had to wait a few years until the 1975 Fair Credit Billing Act gave a green light to cash discounts. Prior to that, credit card companies had refused to renew contracts with merchants who by charging cash customers less than the posted price actually were charging credit customers more to offset service charges. Consumers Union sued American Express to stop the practice and won. Credit card companies currently charge merchants between 2 and 6 percent of their credit billings.

Since then, there have been numerous attempts to institutionalize the idea of cash discounts. Most have been small-time, local operations, and few have been successful. The reasons vary. Among them: Customers object to paying a membership fee, often only second-rate stores participate, merchants fear offending credit card customers, and the programs are poorly advertised and undercapitalized.

Wilson also began with a small-time, local operation in southeastern Missouri. After a year his company, American Business Enterprises, decided to market its Savings Plus card through regional franchises. The card was introduced to St. Louis last August and to Baton Rouge Feb. 1. It will make its debut March 1 in Little Rock and Miami-Ft. Lauderdale, followed by major cities in North Dakota, Nevada, Hawaii, Oklahoma, Texas and Pennsylvania.

Treasurer James R. Hulen said ABE plans to seek franchisees this year in 20 metropolitan areas. One of the areas in which serious negotiations are under way is Washington, D.C. He hopes to begin Savings Plus here by next year.

What makes ABE think it finally has found the key to successful cash discounting? Dave Behnen, the St. Louis licensee, who is also in charge of marketing Savings Plus nationally, cited several essential points: a required savings account, a tie-in with a supermarket, widespread acceptance of the card and selective choice of participating merchants.

First, Savings Plus, like nationally franchised bank cards, is issued free by a well-known local financial institution, thereby assuring its credibility. Second, the cardholder must open at that institution a regular savings account into which the discount is deposited. Behnen believes that enforced savingmakes the customer appreciate discounts more because the money is there at the end of the month, while if a discount is given at the point of sale, the customer merely pockets the change and forgets about it.

A supermarket link reaches a maximum number of people because 98 percent of food shoppers pay cash. (At the discretion of the merchant, personal and traveler's checks as well as money orders also may be defined as "cash.") The ability to use a single card to obtain discounts in many cities is a vital attraction. Finally, ABE will allow only 30 percent of the retailers in an area to honor the card, thus preserving competition. The card-issuing financial institution has a veto over which businesses are selected. Pornography shops, massage parlors and the like are excluded routinely, as are fast-food chains where the typical purchase is too small to make a discount cost-effective.

The size of Savings Plus discounts also is left up to the individual seller and ranges from a low of 1.25 percent on food to a high of 20 percent on specialty gift items. Shoppers frequently get 3 to 4 percent off on appliances, from 5 to 10 percent off on restaurant meals and 12 percent off on beauty treatments. The average discount between 5 and 6 percent bracket, but that varies according to how anxious stores are to get business. In St. Louis, the average is 4.1 percent; in Miami-Ft. Lauderdale, it will be 8 percent.

The organization behind Savings Plus consists of a cooperating bank or savings and loan in each city chosen by ABE, plus a franchisee who signs up merchants, collects discounts and service fees from them, computerizes the discounts and turns them back to the bank for deposit in customer accounts.

Savings Plus is designed to attract more cash-paying customers to a retailer. ABE's marketing study of 3,000 shoppers in St. Louis, Cleveland, Boston, Houston and Sacramento revealed that, although 34 percent of them habitually paid cash in department stores, 71 percent would be willing to do so at a different department store that offered a discount. Overall, 60 to 80 percent said they would patronize another grocery, gift, clothing, drug or furniture store, gas station or restaurant to receive the savings.

In return for instant cash (credit card companies sometimes do not pay merchants for up to three months), the store must pay a service charge to the Service Plus licensee on each transaction according to the type and volume of business. The lowest charge, one-half to three-quarters percent, is paid by supermarkets; and the highest could range up to 10 percent. Three percent is average.

The total cost to the supermarket of the customer discount and service charge thus works out to a minimum of 1.75 cents on every dollar. Behnen remarked that this is "the cheapest promotion a store can go into, much cheaper than trading stamps." For a restaurant, the total cost might well be a 5 percent customer discount plus a 2 percent service charge, or 7 percent. When it was pointed out that some restaurants now decline to accept American Express credit cards because of the 6 percent service charge, Behnen responded that the restaurateur should be willing to pay for instant cash with no possibility of fraud.

He was asked why the restuarateur simply would not decide to give cash discounts on his own and avoid paying the service charge? Behnen replied that the charge also entitled him to free advertising that ordinarilly would cost more than the service fee. More than 1,500 merchants in greater St. Louis have subscribed since last August.

The financial institution picked to issue Savings Plus cards also places an amount of advertising stipulated in its contract. Gary Strong of the Carondelet Savings & Loan Association in St. Louis reported that $30,000 worth of cash card advertising had resulted in 8,000 new accounts. These brought in $5.8 million in new deposits over six months, a substantial sum for an S&L with assets of $560 million. He estimated that four out of five of the accountswere set up to take advantge of Savings Plus. A weekend promotion at a supermarket resulted in 2,500 applicants.

Strong admitted that Savings Plus was off to a slower start than its promoters had predicted. Part of the difficulty has been the reluctance of companies to be guinea pigs, with the result that initial participating merchants are not the leaders in their field. Yet, he said three major airlines have indicated interest once Savings Plus spreads to enough of the cities they serve.

Another difficulty is consumer education. Although all 45,000 Carondelet savings customers have been issued cards, few appear to have used them yet. Strong said December receipts of discounts amounted to $3,800. Assume that 1,000 people used their cards and each got back $3.80. This means the average card holder purchased less than $100 worth of merchandise that month.

While the volume of discounts is not of primary concern to the S&L, which is more interested in total new deposits, it is critical to the Savings Plus franchise which gets its income from service fees. The cost of the franchise is negotiable and factored on the area's disposible income, retail sales and population. Behnen said investors could expect to put up between $50,000 and $300,000. Terms are one-third down in cash, another third after the financial institution is signed and the balance within 18 months. He refused to say what volume is necessary to break even. He also declined to identify the two Washington investors and two S&Ls with which ABE currently is negotiating.

Besides Wilson there are six other partners in ABE. Hulan said the company hopes to go public later this year.