WASHINGTON - 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 -- now selling for 41 cents -- will remain valid for full postage after the increase.

Forever stamps were introduced last year and since then more than 6 billion have been sold. The post office sold $267.7 million in Forever stamps in March, up from $208 million in February and $115 million 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.

Also, for the first time, the Postal Service has stamps available at the new rate before the change takes effect.

Starting May 12, the cost of Forever stamps will increase to 42 cents.

Postage rates last rose in May 2007, a first-class stamp jumping 2 cents.

Although the charge for the first ounce of a first-class letter rises to 42 cents, the price of each added ounce will remain 17 cents, so a 2-ounce letter will go up a penny to 59 cents.

The cost to mail a post card will also go up a penny, to 27 cents. Other increases:

• Large envelope, 2 ounces, $1, up 3 cents.

• Certified mail, $2.70, up 5 cents.

• First-class international letter to Canada or Mexico, 72 cents, up 3 cents.

• First-class international letter to other countries, 94 cents, up 4 cents.

Money orders up to $500 remain $1.05.