THE spiraling costs of postage to nonprofit groups won't make headlines next to stories like the BCCI scandal or the Clarence Thomas nomination. But action by the Senate taken before the summer recess, if unchecked by the House in September, could further erode the important civic infrastructure of US charity and nonprofit groups.Nonprofits are the "thousand points of light" - the educational, charitable, arts, scientific, religious, veterans, fraternal, and political groups that rely on mail for distribution and fundraising. Include us in the mix. Last February nonprofit mail costs were hiked by the postal service some 25 to 40 percent. Given the size of that increase, the House agreed this spring to fully fund next year's subsidies for second and third class postage - $649 million. This will help nonprofits to plan and establish stable budgets. About $40 billion in nonprofit funds are raised each year through mailings - or about $80 raised for every subsidy dollar. That's a pretty good return. Yet on July 18 the Senate only coughed up $383 million in its appropriation. Postal funds must compete for tighter dollars in the Senate subcommittee with the Coast Guard, the IRS, and the Drug Enforcement Agency. What comes next is an appropriations showdown during a joint House-Senate conference in September. Presumably, the Senate wants the House to compromise. That's politics, and it sounds reasonable on the surface. But we believe the Senate should fully fund the nonprofit subsidy. First, many nonprofits are still reeling from the earlier increase. Another hike within a year hardly seems fair. Second, nonprofits have been fighting an annual battle to keep low rates. Congress has seen why the money is important, and has kept the subsidy stable. A second rate increase creates a worrisome precedent. For a decade the White House has extolled the virtues of the nonprofit sector, and has counted on it to contribute to society. Strapped nonprofits may have to cut services which in turn may sap the larger economy. Third, leaving nonprofits to take up slack removes pressure to make needed reforms. Reforms (like those in a recent Postal Rate Commission report) should be given careful attention in public hearings - rather than being rammed through Congress without review as Senator Dennis DeConcini did in July with an amendment claiming to save $180 million. The Senate committee overstepped its jurisdiction. The Postmaster General and Postal Board of Governors disagree with the alleged savings. The House authorizing committee should nix the amendment. Nonprofits deserve to be shielded from both the arbitrariness of Congress and the market - though remaining responsive to fiscal needs. The alternative is undisciplined rate increases whenever nonprofits clash with the pet funding causes of lawmakers. Some suggest a new subclass of nonprofit mail. Others say the Post Office should take over from Congress (an idea with its own dangers). Whatever the strategy, it must be remembered that nonprofit mail supports a common good. It deserves support.