The U.S. Postal Service has issued a five-point rebuttal sheet to harried window clerks, telling them how to deal with customers who complain about the latest (2 cent) increase in the price to postage stamps.

During the weekend, the price of a first-class stamp went from 20 to 22 cents. Because most post offices were closed Sunday and Monday, yesterday was the first chance citizens had the eyeball-to-eyeball chance to say what they thought about the situation as they bought new stamps, or 2-cent models to upgrade their existing supplies of the 20-cent version.

Postal Life, the official magazine of the Postal Service, advises employes to use the "five verbal Rolaids to sooth the discomfort that can result from sudden swallowing of a rate increase. Administered with sympathy, they'll go a long way toward making a change in the cost of postage easier to digest."

When customers complain about the new stamp price, employes are advised to tell them:

* The price of the first-class stamp was unchanged for 3 1/2 years. "How many other products and services can you say that about?" The cost of living is up 13 percent since November 1981, the Postal Service said, but the stamp price has only now gone up 10 percent.

* Take advantage of the "candy bar syndrome." Tell customers that in 1969 the Hershey bar cost 10 cents for 1.5 ounces. "Today it costs 35 cents for 1.45 ounces," the Postal Service says. That does not mean people should eat stamps. What it means, postal officials say, is that ounce for ounce the first-class stamp is a better deal than a Hershey bar, particularly when you want to mail a letter.

* If you lived in Norway, clerks are advised to tell customers, you would pay 29 cents to mail a first-class letter. West Germans pay 27 cents, and Canadians 24 cents.

* Until 1970, the postal service says, first-class stamp users helped subsidize other classes of mail. Now the 22 cents you stick on each letter represents the actual cost of mailing that letter.

* First-class isn't the only rate that has gone up. Some other classes of mail have gone up as much as 13.8 percent.