Getting a letter, package or even junk mail this summer might require you to go to the nearest post office, cross a picket line and line up with other customers at the general delivery window. That is one of the grim prospects if the U.S. Postal Service is hit with a strike by its 500,000 rank-and-file workers.

Contract talks between the USPS and union leaders begin next month. But already militant local union leaders are predicting an illegal, wildcat walkout unless they get pay raises, cost of living increases and job protection at least equal to those provided by their current contract. That three-year agreement expires in July. An estimated 8 of every 10 employes belongs to at least one union.

USPS officials and national union leaders are playing down any strike talk. But this column has obtained copies of local and regional USPS strike contingency plans. They indicate that management plant to try to move the mail - and take disciplinary action against strikers - if a walkout should occur.Existence of the contingency plans themselves does not indicate that USPS has given up hope for settlement; it is merely to make sure the USPS is ready for anything.

During the Johnson administration, postal officials made a secret study that predicted "severe damage" to the nation's economy even from a strike of short duration. It would be worse, obviously, if a strike hit around the first of the month, when more than 64 million people regularly get pay or pension checks by mail.

Since that study, mail volume has increased. But many firms have switched to the electronic transfer of funds to banks. The government, however, still mails social security payments that go to 1 of every 7 persons in the nation.

On March 13, this column outlined a local postal strike contingency plan. It instructed postmasters to borrow federal troops, national guard and college ROTC unit members to help sort and move the mail. The plan also calls for dropping Saturday service in areas where manpower is short.

Although postal unions are not prepared for a strike (strike funds are almost nonexistent and experience in managing a strike was limited to a wildcat strike walkout a few years back), money and political pressures make possible a strike that neither side wants.

Nearly all major postal union leaders are up for reelection this year. They are under tremendous pressure from members (and challengers who want their jobs) to better the current contract. Union officials say the average employe today gets about $15,000. Postal officials say it is closer to $17,000.

Postal officials, who are trying to run the USPS like a corporation, say they must be allowed to automate some operations and initiate other economies that would eliminate jobs. The USPS already has asked permission to raise the price of a first-class stamp this year from 13 to 16 cents.

Legislation now before the House would revamp the USPS and dramatically increase taxpayer subsidies to hold down rates and stave off service cuts. The Carter administration is opposed to the legislation (HR 7700). Postal unions favor it.

Copies of a national strike contingency plan obtained by this newspaper show that postal officials are prepared to stop home delivery if a strike makes it necessary.

The USPS strike - contingency plan also spells out temporary procedures that nonstriking employes and supervisors would use to sort mail. This includes preparing it alphabetically (for window pickup) rather than by address for home deliver. It also spells out security measures local officials would take to protect employes who continued to work, buildings housing postal facilities and plans for guarding or moving postal vehicles to areas where the danger sabotage would be reduced.

Officials also are advised to keep track of workers who walk off the job during a strike action, and not to approve automatically sick leave absences if large numbers of workers call in during a strike. It also reminds supervisors and employes that the penatly for striking the government can be a $1,000 fine, a year and a day in jail and dissmissal.

Tape Gaps: Blank spaces on tape-recordings continue to plague official Washington. On March 10, Labor Secretary Ray F. Marshall addressed the Newspaper Publishers Association.

Marshall's speech, from the official transcript, went well up until Page 12 when he started talking about unemployment. It ended like this: "As you know, unemployment has declined steadily from the time that we came in; it declined over one full percentage point during 1977, and gives evidence of continuing to do that. With the performance of the economy last year - (remainder inaudible due to poor tape)." Where is former senator Sam Ervin (D-N.C.) when we need him?