The U.S. Postal Service yesterday proposed giving large-volume first class mailers a half-penny discount as an incentive for using the controversial nine-digit Zip code.

The proposal, which is scheduled for consideration by the Postal Service's board of governors next month, must be approved by the independent Postal Rate Commission before it become effective.

To qualify for the discount, businesses would have to mail a minimum of 500 first class letters or cards at a time, used the expanded Zip code, afix address labels that can be read by the Postal Service's computers, and fit into one of the Postal Service's three technical mailing categories.

For example, large-volume mailers using the expanded Zip code who sort their mail in bundles down to the first five Zip code numbers before mailing would pay 12 1/2 cents per letter instead of 13 cents now charged, postal officials said.

A vote on the incentives will be the board of governors' first opportunity to technically approve the nine-digit Zip code, although Postmaster General William F. Bogler has lobbied for months for public approval of the Zip codes and plans to put them into effect later this year. The governors already have approved the purchase of computers to read the longer Zip codes but the machinery also can be used for the five-digit Zips, postal officials said. Approval by the governors wasn't necessary to implement the expanded Zip code.

Expanded Zip codes already have been assigned, but won't be verified until June, said James Finch, senior assistant postmaster general.

However, legislation prohibiting the Postal Service from spending any money to implement the extended Zips is pending in Congress.

The board of governors also said it would have no decision on the Postal Rate Commission's recommendation to raise the price of a first class stamp from 15 cents to 18 cents and the price of a postcard from 10 cents to 12 cents. Board chairman Robert Hardesty said the board wouldn't have a rate decision for at least a week.

Bolger had requested increases from 15 cents to 20 cents for a first class stamp and from 10 to 13 cents for a postcard in addition to different rates for other classes of mail.

Bolger warned last week that if the board of governors accepts the rate commission's recommendations, he may have to ask for another rate increase in the next few months.