The U.S. Postal Service today ordered a liberalized parcel post system that would allow thousands of new business customers to use bulk mail.

The system would make it economical for department stores, for example, to mail merchandise to customers instead of making deliveries with their own trucks.

The USPS Board of Governors passed the interim measure, to take effect at 12:01 a.m. Feb. 25, at the board's quarterly meeting here.

Postmaster General William F. Bolger said that the new classification is intended to revive a losing U.S. parcel post system, which is being beaten in its own game by commercial deliverers, chiefly United Parcel Service.

He said also that a winning parcel post system might offset to some degree an expected half-billion dollar deficit in the mail system in the 1980 fiscal year.

An actual surplus of $180 million is expected this fiscal year ending Sept. 30, but that black-ink situation -- the first since 1947 for the government mailing system -- would turn around because of increased labor costs.

A USPS request to reclassify the parcel post system for increased cost bulk mail volume is pending before the U.S. Postal Rate Commission, but federal law allows the board of governors to implement the system temporarily until the commission reaches a decision. That is expected by midsummer.

The old regulations allow bulk mailings for 300 or more parcels, each weighing at least 16 ounces. The new plan would reduce the minimum to 50 parcels at eight ounces each.

Bolger said that in other mail classifications, volume has increased to a point that the postal service will be handling 100 billion pieces of mail in the 1980 fiscal year.

"The only decrease has been in the parcel post area," he said. "We've been running behind for years. After we increased our parcel post rates by 35 percent, we experienced a drop-off in volume by 27 percent."

In the rate request, which is being opposed by United Parcel, a USPS economist testified that average postal rates for parcels in the two-to 50-pound weight range are 25 percent higher than United Parcel's rates.

"Differences as large as $1.16 for each five-pound parcel make it economical for even low-volume business shippers to use UPS and to pay for weekly pickup charges," he testified before the commission.