THE POSTAL SERVICE is taking the first steps toward putting into effect the new ZIP Plus Four system by telling big users of the mails exactly how it will work. That will probably be enough to reactivate the "Zap the ZIP" lobby, even though the Postal Service is barred by law from making the new system effective, even on a voluntary basis, until late 1983. The zappers almost persuaded Congress last spring to kill the nine-digit ZIP before it got even its first number unveiled.

In case you haven't been following the develop ments in this long battle, it involves the Postal Service's claim that by adding four numbers to existing ZIP Codes it can save hundreds of millions of dollars in each future year. Its prime target is large business mailers, who generated 84 percent of the 106 billion pieces of mail delivered last year. The struggle has taken many peculiar turns, largely because it is a mixture of economic data and gut reaction.

The case for the nine-digit ZIP is that it will permit the original sorting of mail to expand from postal zones to carrier routes and below. Performed first by people and later to a large extent by optical scanners, this procedure would eliminate a costly sorting step and help keep the price of sending a letter within the reach of middle-class citizens.

Some opponents of this proposal--which will cost the Postal Service almost a billion dollars to put into effect--argue that the savings are exaggerated, the equipment won't work, and the costs of conversion by large business firms (the really big mail users) have been vastly underestimated.

We're not about to get into that argument--although on the surface the Postal Service seems to have the better of it--because the other complaints about ZIP Plus Four are the ones that many congressmen don't seem able to handle. These include such arguments as that the addition of four digits to the ZIP Codes will turn America into a numerical state, that nobody can remember nine digits (who can remember five, for that matter?), and that it is some how immoral for the Postal Service to move one step closer to changing 79 Wistful Vista to 69874-4851. The Zap-the-ZIP forces have parlayed such ideas into thousands of letters mailed to Capitol Hill.

Our emotions are with the zappers--Fibber McGee's closet wouldn't have been the same at 69874-4851 as at good old Wistful Vista--but all the logic runs the other way. The Postal Service desperately needs to use every tool at its command to cut the labor costs of sorting the mail. The telephone system was infinitely more graceful in the days of Capitol 5 and Republic 7, or, before that, when you asked the operator for "85." But the price of telephone calls has stayed down because of innovation and mechanization. The Postal System has to do the same, whether with this system or some variation as ZIP Plus Four--or the day will come when a stamp costs about as much as a steak.