The Postal Service is reducing some rates for high-volume customers.

The stamp with the discount is a 21.1-cent coil, which shaves virtually a penny off the first-class postage rate. It is being put out Tuesday in Washington in conjunction with the annual meeting of the Envelope Manufacturers Association. Postmaster General Paul N. Carlin, who does not often preside over the debut of coils, will be the principal speaker at the first-day ceremony at the Hyatt Regency Capitol Hill Ballroom.

The 21.1-cent denomination meets the first-class rate for mailings of a minimum of 250 pieces that bear nine-digit ZIP Codes but are not presorted according to the nine-digit code. There are also some technical requirements, such as mail size.

In addition to this .9-cent discount, there are plans to issue a 17.5-cent coil -- a discount of 4.5 cents -- for some 1,200 big mailers that presort their nine-digit ZIP Code mail.

The savings can add up to a bundle for the users of these stamps, who make mass mailings. The discount is granted by the Postal Service because it saves money in the sorting process when the nine-digit ZIP Code is used.

Six billion pieces of ZIP-plus-four mail were processed by the Postal Service in the just-ended fiscal year 1985, triple the number of the previous year. Use of the expanded ZIP is expected to save $900 million a year in labor costs when fully implemented.

The new stamp is noteworthy because it is introducing a number of philatelic firsts that could become routine in the future.

With this stamp a new precanceling format is being introduced, as is a postal rate classification never used before even on the many decimal denominations in the Transportation Series coils. The issue is the first to use a new kind of paper developed for stamp printing. It is the first American multicolor gravure coil stamp and the first to have as many as six plate numbers on each 24th stamp in a coil.

The design has the outlines of five envelopes, edged in pink, red, yellow, green and blue, descending like a staircase from left to right. Across the back of the bottom envelope is a large "ZIP + 4" precancellation that fills a third of the stamp.

The stamp is being produced in non-precanceled form in coils of 500 and in precanceled form in coils of 500 and 3,000.

Collectors of first-day-of-issue cancellations have alternative ways of ordering and a deadline of Nov. 21 for sending in orders.

Collectors affixing the new stamp on their first-day covers, which must be addressed, must add an additional 1-cent postage for the first-class rate. The envelopes should be sent for servicing to Customer-Affixed Envelopes, Letters Stamp, Postmaster, Washington, D.C. 20066-9991.

Collectors preferring full processing by the USPS should send their addressed covers plus 23 cents for each stamp to be affixed to Letters Stamp, Postmaster, Washington, D.C. 20066-9991. A one-cent Omnibus stamp will be added.

An 18-cent coil, delayed for three months by preproduction problems, is being issued Nov. 6. The Postal Service said the stamp will serve as the basic rate for presorted first-class mail presented at a post office as part of a mailing of not less than 500 pieces.

The stamp bears a Gilbert Stuart portrait of George Washington flanked by the Washington Monument. It has a new big precancellation, "Pre-sorted First-Class," the new thinner paper and all the other characteristics of the 21.1-cent including rolls of coils of the same quantities.

Collectors of first-day-of-issue cancellations must send in their orders postmarked no later than Dec. 6 and have the usual different ways of ordering.

Collectors acquiring the stamp themselves at post offices must add an additional 4 cents in postage to meet the first-class rate. Orders should be sent to Customer-Affixed Envelopes, Postmaster, Washington, D.C. 20066-9991. Covers must bear return addresses.

Those wishing complete servicing by the USPS should send their addressed covers to George Washington/Monument Stamp, Postmaster, Washington, D.C. 20066-9992. The Postal Service will add a 4-cent Stagecoach stamp to the 18-cent coil. The cost is 22 cents for each envelope submitted. Personal checks are accepted and desired. Stamps T