With a postal rate increase just two weeks away, Americans are buying 30 million Forever stamps a day.
The cost of sending a first-class letter will rise a penny to 42 cents on May 12, but the Forever stamps - currently selling for 41 cents - will remain valid for full postage after the increase.
Forever stamps were introduced last year and since then more than six billion have been sold, with interest growing as the rate increase nears.
The post office sold $267,696,023 in Forever stamps in March, up from $207,900,132 in February and $115,303,031 in January.
Unlike the Forever stamps, other 41-cent stamps will require additional postage when the new rates take effect, and postal officials said they printed an additional 1.5 billion 1-cent stamps in anticipation of the demand.