THE LATEST example of why it's wrong for Congress to legislate in last-minute megabills has to do with the mail. The Postal Service has announced that, because of the budget cuts imposed in last month's reconciliation bill, post offices will be closed an extra half day each week.

Is this a bad idea? We don't know; it may be something the service should have done on its own years ago. But we do know this: to some extent the budget cuts are phony, and if Congress -- and the Office of Management and Budget, also a player -- did the right thing, it was by accident.

The story begins with the budget summit. The summiteers couldn't agree on enough tax increases and major spending cuts to get the deficit as low as they wanted. To make up the remainder, they put several arbitrary items in the agreement, goals more than clear ideas. One was an order to the civil service committees to save $850 million this year and next in personnel reforms.

Some summiteers thought the committees might be able to do this in part by freezing longevity or so-called step increases in pay. For various reasons -- including a Catch-22 in the complicated rules by which budget cuts are "scored," an art form in itself -- it turned out they could not. The summiteers also believed the committees could save some money by stretching out the right of retirees to lump-sum payments from the civil service retirement fund -- and the committees finally did this. But this is a gimmick, a timing change rather than a real reduction in federal obligations.

The only other ideas anyone had involved the Postal Service. Though rendered independent in 1970, the service continues to have many obvious ties to the government. One is that the Treasury bears some postal retirement costs. The committees proposed shifting these costs to the service, which, if it chose, could then seek an increase in the price of a stamp to foot the bill.

A stamp increase is the ultimate user fee, and you might think the administration would like it. But OMB objected. It said the proposal violated the summit agreement, which had been to achieve the personnel savings through a spending cut; this would be a revenue increase instead.

The service and its champions in Congress said OMB was intruding where it did not belong. The issue became the more complicated one of how independent the service really is, and should be. The service says all it wants is to be left alone, that it would gladly pay the full retirement costs and even give up its right to borrow through the Treasury if it could have financial and operating independence in return. Turn it loose, it says, and OMB shouldn't care either what it spends or how it raises its money.

But at least for fiscal purposes the administration says this would perpetuate a fiction. Its view is that the service remains a federal entity, with a statutory monopoly (on first-class mail) and statutory powers and duties; that it and its powerful unions have their own source of quasi-public funds, through the cost-plus process by which postal rates are set; and that its spending and borrowing are part of the federal impact on the economy each year.

The administration says it would mask this impact to move the service back off budget, where it used to be and wants to return. It believes there should continue to be some control. That is the many-tiered issue that animated and went undebated in this fragment of the reconcilation act. The hasty result was inconclusive legislation under which the service is not even obligated to pay the disputed retirement costs beyond this year. The issue will have to be dealt with again in an orderly way. That, of course, is how it should have been dealt with in the first place.