Mom-and-Pop stores used to be the backbone of retailing in this country.

Grandpa bought everything he needed from them.

But Grandpa and the Mom-and-Pop store are both extinct now. New business machines and modern business techniques have changed retailing into a fabulous wonderland of efficiency. Or something.

A few days ago, I went out to buy some house-brand mouthwash that my wife uses by the gallon. It is not germicidal, but my wife likes it and I buy it to keep the Peace. Naturally, I wait until it is on special, at about 15 per cent off.

When the "special" ad ran recently, I went to one of the stores in the chain and bought out its entire stock. At the checkout counter, I said, "These are on a two-for special. I bought three bottles because that's all you had."

The clerk frowned and tried to figure out how to ring up three bottles at a two-for price. After a while, her face brightened and she announced, "You can't have three at the two-for price. You have to buy two or four."

"Lady," I said, "I bought all you had, and I'd like a rain check for six more."

"I'm sorry," she said, "but you can't buy three, and our limit is six. So if you take two now, you can get only four more."

"Your ad doesn't say nothing about a limit of six," I protested.

"Oh, yes it does," she said, pointing. "See. Right here."

I put on my eyeglasses and read some tiny type that said the management reserved the right to limit quantities. Not quantities on some specific item, but quantities on all items. No specific limits. Just a general disclaimer that freed the management from any obligation to sell a customer more than one of any advertised item.

The Federal Trade Commission has issued some vaguely worded decrees banning this type of advertising. But enforcement is almost nonexistent and all the major drug chains here use the "reserve the right to limit quantities" language in their small print.

Inasmuch as there is nothing to be gained from arguing with modern business methods, I paid for the two bottles of mouth wash and left.

Later in the day, I passed another store in the chain and stopped to see if it had any house-brand mouthwash. I found acres of it on the shelves!

As I pondered this development, the store manager walked by. "Is there as limit on how much of this mouthwash I can buy?" I asked.

"Why would I put a limit on it?" he replied. "I'm here to sell it, not look at it. Buy as much as you want."

Another local drug chain offered a special on four-bar packages of Ivory soap recently. When I got to the store I examined a few packages to determine whether the 4-for-49 price was still in effect. Finding no prices on the first few packages, I asked a cashier. "They're still 4-for-49," she said.

I put about 10 packages in my cart and took them to the cashier. She noticed that some were unmarked, some were marked 49 cents, som 59 cents, and some 69 cents. Crisis!

She called for the manager, but it turned out that he knew less than she did. After he had held up the line for about five minutes as he tried to decide this complicated problem in merchandising, he ruled: "You have to pay whatever is marked on each package. The ones that are marked '49' you get for 49 cents, the ones marked '59' will cost you 59 cents, and the ones marked '69' will cost you 69 cents." In silence I held up an unmarked package, and he added, "You can have the unmarked ones for 59 cents."

"If King Solomon had graduated from Harvard Business School he couldn't have made a wiser decision," I told him as I handed him the entire sack of soap and headed for the door emptyhanded.

Before I could get out of earshot, I heard the cashier calling after me, "Aren't you buying the soap? I rung it up already. Wait a minute, mister. You have to put your name and telephone number on this register receipt that I'm voiding. It's store policy, mister. Mister . . ."

"I know about your store policy," I said, "but I it is my policy not to sign a register receipt that somebody can misconstrue as evidence that I received a refund from you. And I don't give out my telephone number to every pretty girl who asks for it. That's my policy, and you're stuck with it, just as I'm stuck with yours."

She muttered something, and I left wondering how the Moms and Pops who were so good at pleasing customers could have produced so many computer-assisted merchandising bumblers.