AT FIRST glance it seems like someone decided to ruin one of the four stamps in Canada's set of 1987 Christmas stamps. There, next to the brightly colored presents under the Christmas tree on the 31-cent stamp, are four vertical black bars, an almost garish element not on the other three stamps in the series.

But it's not an error. In fact, it's the key to an idea that a lot of holiday mailers probably wish the U.S. Postal Service would adopt.

The bars on the 31-cent stamp are used by electronic processing equipment to help scan the addresses and speed processing of the letters.

The bars also signal a discount that Canada Post Corp. offers to holiday mailers who use the stamps and send their holiday greetings in envelopes with specially printed postal code boxes. Individuals who use these envelopes and the stamps with bars get to mail their holiday greetings at five cents below the domestic rate.

It's the second year that Canada has offered the discount and the idea, called the "Greet More" program, seems to be working "quite well," says Deborah Saucier, a spokeswoman for Canada Post.

There were some problems last year as many holiday mailers failed to mark the proper postal codes on their preprinted envelopes. This year, with 30 greeting card companies participating, Saucier said she expects the program to win greater acceptance. The greeting card companies have promised special instructions in each box of holiday cards in an effort to eliminate confusion, she said.

The discount program was triggered by surveys that showed since 1981 fewer Canadians were sending holiday greetings and that the villain was the country's higher postal rates. Since holiday mail accounts for a lot of postal revenue in Canada, postal officials there decided to enter the discount trade and win back their customers.

Saucier says that Canada Post figures that it saves two cents on every preprinted envelope it processes and it figures that about 40 percent of the 100 million pieces of holiday mail it handled last year was at the discount rate.

This year's set of Canadian holiday stamps features the work of Quebec painter Claude A. Simard. The stamps were issued Monday and come in denominations of 36 cents, the domestic first-class rate; 42 cents, the rate for U.S. mail; 72 cents, the rate to most overseas countries; and the special 31-cent stamp. The special stamp is offered in booklets of 10 stamps and can be used for mailings at the special rate until January 31.

Currently, only bulk mailers in the U.S. can qualify for discounts below the current first-class rate of 22 cents per envelope. A spokesman for the Postal Service said this week he knew of no plans to offer discounts to individual mailers similar to those offered in Canada.

Canada, like the U.S., used to offer holiday discounts to individuals who would not seal their holiday envelopes, but that discount was abandoned years ago in both countries.

Robert M. Gibbs of Saratoga, Calif., a San Francisco suburb, recently went to London where he sold his collection of stamps from the former British colony of Rhodesia for more than $1.1 million, the Associated Press reports.

Gibbs, reported to be a 41-year-old accountant and real estate developer, had a collection that included issues dating to days when the country now known as Zimbabwe was run by the "British South Africa Company" and its stamps bore the company's name.

Top price at the auction was $129,000, paid by an unidentified collector for a set of 22 proofs of engravings of early stamps for the colony by the British printers, Waterlow and Sons.

It's a rare collector who doesn't dream his collection will someday be valuable and Eric E. Erdmanis of Havertown, Pennsylvania, may have found the secret.

Erdmanis reports that he has received six first-day cancellations of the recent Constitution commemorative on $100 bills. They make the perfect first-day cover, he reports.

Ben Franklin, who signed the Constitution is on one side of the bill and Independence Hall, where the document was signed, is on the reverse.

Postal officials have announced that Johns Hopkins, the man who gave Baltimore $7 million for a hospital and university, will be honored with a stamp in its Great Americans series.

Bill McAllister is a member of The Post's national staff.