Commercial users of the U.S. mail developed plans yesterday to do business without the Postal Service's help as the service came no closer to averting a strike threatened at midnight tonight.

The plans include:

American Express intending to advise credit card holders that they have to pay their bills anyway and that they should go to the nearest American Express office to do it.

Emery Air Freight Corp. gleefully planning to increase its sales staff by 100 percent.

The Walls Street Journal advertising that subscribers will have to go to newsstands to pick up copies of the paper.

Meanwhile, negotiators for the Postal Service and unions representing 550,000 postal workers remained far apart on several major issues - including job security - as federal mediators probed for a consensus that would enable full-sclae bargaining to resume.

Wayne L. Horvits, the Federal Mediation and Conciliation Services chief, reported some progress on other issues last night and said talks would continue "night and day."

Postal Service spokesmen said that if a strike occurs, an attempt would be made to deliver first-class mail' including Social Security checks, veterans' checks, government workers' paychecks, tax returns, retired civil service employes' check and perhaps a letter from home.

But there would be new mailmen: postal service supervisors and - if there weren't enough supervisors to go around - "the military could deliver the mall but that's up to the president," a postal service spokesman said.

Newspapers, magazines and advertising circulars would pike up at the post office until the strike ended, when it would descend on "resident."

Some companies do not trust the Postal Service word that the mail - or at least the first-class segment of it - must go through. They say that any mail strike results in delays in service. So they have tried to find mail substitutes.

American Express, for example, probably will not penalize credit card holders if their payments arrive late, but the company intends to take out advertisements in newspapers asking clients to deliver their checks in person.

Washington Gas Light intends to send its workers throughout the District delivering bills. The company mails 25,000 bills each day and does not want to get any more behind than necessary.

Bache & Co., a brokerage firm, stopped sending mail Tuesday night because of fear that it would languish in the post office during a possible strike. The company plans to use couriers to deliver and receive orders.

The courier firms, United Parcel Service and the air freight companies, say they will gladly pick up any slack the Postal Service leaves.

"If you're going to be evicted tomorrow and you have to get your rent to the real estate office right away, you might want to use our service," said Ann Christian, manager of one courier company in the District.

Emery Air Freight Service intends to increase its sales staff and rent additional trucks to accommodate increased business that a strike would bring.

"If someone is applying for a patent that could make him a millionaire," he probably wouldn't want to depend on the U.S. mail during a strike," said George Pakidas, service manager. "He'd pay us $20 and we'd get his package there in two day."