THE FIRST GREAT airlines discount sale ends Sunday. That's when United and American Airlines will stop giving every passenger a coupon worth half the price of another flight later this year. The coupon gimmick seems to have done wonders for United. Faced with the prospect of flying mostly empty planes when it returned to the air late last month after a long strike, United offered the coupons to entice its former passengers back. American quickly matched the offer, and the early traffic counts show both airlines did a land-office business for three weeks. It was almost impossible at times to get on United's shortest (and cheapest) flight from Norfolk to Newport News; people were apparently taking the flight simply to get the coupons. But no one will know for sure until the end of the year whether the offer stimulated enough new business to cover the revenue losses when the coupons are redeemed.

The coupons, of course, are only part of the price-cutting war that is under way. Before the DC10 investigation grounded its fleet, World Airways was making substantial inroads into the coast-to-coast traffic with a $107.99 one-way fare. To meet that, United, American and TWA offered a $108 ticket. And World promptly challenged the fare before the Civil Aeronautics Board as "predatory."

All this is very much in the tradition of the old price-cutting wars between discount stores. And it is going on all over the place. North Central joined in this week with a round-trip ticket from Washington to Minneapolis for the price of a one-way ticket plus five cents. Then, of course, there was the single day earlier this year when Texas International offered flights from Washington to Dallas for 99 cents.

But prices and sales, as every good shopper knows, are not always what they seem. Some airline fares are quietly going up. Some of the bargains carry with them a good deal of fine print about when they can be used. And some of the highly advertised super-bargains are not as great a departure as they may seem from what was already available.

What you're witnessing is a classic example of free enterprise. The bargains exist where competition for passengers is sharp and where airlines are trying to open routes they have not served before. That's what the advocates of deregulation had hoped would happen when Congress and the CAB started deregulating the airlines. They also hope it will continue to happen and not end suddenly when weaker airlines are driven out of key markets and the survivors act like monopolists and raise prices. But that, too, remains to be seen.

In the meantime, the summer of the gasoline shortage may also go down as the summer of the airline sales (despite the grouding of a major plane). If it does, Congress can take credit for both.