WHATEVER ONE may think of the postal unions' demands or the U.S. Postal Service's response to them, the eleventh-hour agreement last week on a procedure for resolving the current contract dispute - and avoiding an immediate mail strike - is welcome news. Chief federal mediator Wayne L. Horvitz, who is credited with hammering out the agreement, acted swiftly to combine the unions' demand for more negotiations with the management's call for arbitration. That's the same sensible procedure used before in rail, longshore and other disputes.

Fortunately, leaders on both sides of the dispute recognized some face-saving gestures in the proposal by Mr. Horvitz, who was careful to characterize the procedure as a "continuation of the collective-bargaining process."

The plan has set up a resumption of bargaining with a special mediator and, if no agreement is reached within 15-day period, the mediator will decide any remaining unresolved issues. Obviously, that doesn't rule out the possibility of wildcat stikes. And already one ambitious local president in New York, Moe Biller, is second-guessing national leaders and muttering wildcat, incantations. There are laws to handle whatever job actions Mr. Biller may suggest to his followers - just as there are taxpayers who may not take so kindly to postal employees who decide to circumvent time-honored, agreed-to, collective-bargaining arrangements.

A deal is a deal. If Mr. Biller or other self-appointed critics of negotiated arrangements choose to show off and hit the bricks, it must be at their peril. Moreover, the government must be ready to act - with the military, private services or whatever it takes - to step in and replace any employee who choose to ignore an agreement that AFL-CIO president George Meany has hailed as "in the true tradition of collective bargaining and trade unionism."

Most thoughful leaders of government employee unions recognize the value of procedures aimed at averting stikes - for the sudden, unannounced disruption of essential services can threaten the health and/or public safety of innocent bystanders. That is why we welcome the latest agreement toward a postal settlement as an example of union-management responsibility.