GREEN BAY, Wis. (AP) -- It'll become a penny more expensive to send a first-class letter starting tomorrow.

So people are buying up stamps ahead of time, before the cost goes to 42 cents.

The Green Bay Post Office says that there's been an increase in demand for Forever stamps. Those stamps cost whatever the going rate is -- and they can be used, just like their name, forever. No matter how much postal rates rise.

Green Bay Post Office's Terri Michlig says people like that they can use the stamps even a decade from now.

Other stamps will require a 1-cent stamp starting Monday.

The U.S. Postal Service issued 5 billion Forever stamps to meet increase demand before the price change. More than 6 billion have been sold since they were introduced last year.

The rate increase won't be the last. 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.