WASHINGTON—The cost of mailing a letter goes up a penny to 42 cents on Monday, the latest in what are expected to be annual price adjustments by the Postal Service.

A new law regulating the post office makes it easier to raise rates as long as the agency doesn’t exceed the rate of inflation. Rates are to be adjusted each May.

But the post office also has introduced a way for people to save money when the price goes up, the Forever stamp, which remains valid for first-class postage regardless of any increases.

With the rate increase approaching, sales of the ...