IF YOU STILL think of the Postal Service as a hopelessly inefficient agency with large losses and little future, think again. The service has just announced that it expects a revenue surplus for this fiscal year -- the first since 1945. That good news comes with a few caveats. The gain, probably around $400 million, includes the $920-million-per-year general federal subsidy. Moreover, rising costs -- especially cost-of-living increases in wages -- are expected to push the service into the red again next year and dictate another round of rate increases in 1981. Still, the central fact remains: the Postal Service is doing much better than most people expected a few years ago.

There are two major reasons for the gains. One is better management. Postmaster General William F. Bolger and his predecessor, Benjamin F. Bailar, have gotten a better grip on payroll costs -- which consume 86 percent of the budget -- by improving productivity and offering discounts to large-volume customers who sort their mail themselves. Second, contrary to many predictions, private delivery services and electronic communications networks have not yet taken away huge chunks of postal patronage, except for parcel post. Meanwhile, catalogue shopping is booming. Thus the volume of mail, which dipped to 89.5 billion pieces in fiscal 1976, is growing steadily and may exceed 99 billion pieces this year.

The future holds problems. There are the constant problems of containing costs and maintaining the expensive public services, such as rural delivery, which a national postal system must include. Beyond that lies the new era of communications, which is approaching so rapidly and may soon enable many postal users to transmit most of their messages and money electronically. Mr. Bolger is determined to compete for this business -- and that has plunged the Postal Service into a fierce regulatory battle with private communications companies.

Ultimately, Congress will probably have to intervene in this multi-billion-dollar fight. So far, however, the complexity of the issues and the pace of technological change have frustrated congressional efforts to rewrite the communications laws. It has been easier for the lawmakers to poke at familiar issues, as the House did last week when it passed a bill granting the Postal Service larger subsidies -- which the agency did not request -- and making the postmaster general a political appointment once again. It is discouraging to see the House backing away from the concept of a businesslike postal system, just when that system is finally moving ahead.