With postal rates set to increase Sunday, it seems that some businesses have found a way to beat the high cost of mailing -- at least when it comes to mailing overseas. Postal investigators have compiled a list of corporations that send their executives overseas with suitcases stuffed with letters and parcels to be "remailed" in a country with lower rates.

There's only one problem with the scheme: It could be illegal, because it may be a violation of the U.S. Postal Service's mailing monopoly. The postal investigators have turned over their list to the Justice Department, according to a Postal Service spokesman, and now Justice is investigating the matter.

The Reagan administration has never been particularly fond of the Postal Service's monopoly but could wind up defending it in court.

According to postal insider Van Seagraves, publisher of the Business Mailers Review newsletter, the list of offending firms "is almost a who's who of American business." TO METER OR NOT TO METER . . .

The government-wide cost-cutting crusade has come around to those prestamped envelopes marked "Penalty For Private Use" stacked in every government office.

The Postal Service is telling federal agencies that they can reduce their postal costs by using postal meters instead of the imprinted envelopes for at least some of their mail.

Under the current system, agencies twice a year count the imprinted mail sent over a two-week period. The Postal Service then averages that figure to project an agency's costs for the next six months. With meters, postal officials say, agencies will pay only for what they use.

But some postal officials are concerned that the switch could threaten the service's cherished monopoly. These officials fear that once agencies are told to account for all mailing costs, some might be tempted to switch to cheaper, competing carriers and messenger services.

Richard Stephenson, chief of mail and correspondence management for the General Services Administration, thinks the new system may be costly and ill-advised. "It doesn't guarantee any more accurate postage" to use meters, he said. "A meter does not guarantee the correct postage -- it does what a human tells it to do." He said humans using postage meters inevitably make mistakes, putting too much postage on some letters and packages, adding to waste.

The other reason for the switch is to stop employes from using the imprinted envelopes for personal mail. But, Stephenson said, the potential for abuse remains, because once a letter is sealed, only the sender and the recipient know what's inside. NEW FACES. . .

The Postal Service has announced personnel changes, generally promoting some long-time employes, in the first major shake-up in the top ranks since Postmaster General Paul Carlin came on board at the beginning of this year. Michael S. Coughlin, an 18-year postal veteran, has been named senior assistant postmaster general of the employe and labor relations group; Jerry K. Lee, a 26-year-veteran, will be promoted to senior assistant postmaster general for the research and management systems group; and William B. Cummings, will move from the planning department to become senior assistant postmaster general in the finance group.

Changes in the field were also announced, because both Carlin and his deputy-designate, Jackie A. Strange, came to Washington after heading regional offices. Fletcher F. Acord, a district manager for the western slopes district in Utah, will replace Carlin as regional postmaster general for the central region, based in Chicago. And Harry C. Penttala, will replace Strange as chief of the southern regional office in Memphis.