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 — 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.

"We knew the Forever Stamp would be a big hit with our customers and we continue to replenish our stock to meet demand," said Postal Service Consumer Advocate Delores Killette. "We introduced these stamps as a customer convenience to ease the transition during price changes."

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.

A set of five 42-cent stamps honoring pioneering journalists went on sale last week, as did a set of four stamps featuring the American flag flying at different times of day.

But don't wait forever, because starting May 12 the cost of Forever stamps goes up to 42 cents too.

And even as the higher rates near the post office, like most Americans, is seeing higher gasoline prices eat away at its budget.

It has been estimated that each penny increase in the price of gas costs the post office $8 million-a-year.

Postage rates last went up in May 2007, with a first-class stamp jumping 2 cents to the current 41-cent rate.

The first-class postal rate is the one that most people notice, but other prices will also rise.

In the past raising postage rates was a long, complex process involving hearings before the independent Postal Regulatory Commission, a process that could take nearly a year.

But under the new law regulating the post office that took effect in late 2006, the agency can increase rates with 45-days notice as long as changes are within the rate of inflation for the previous 12 months. The Postal Regulatory Commission calculated that at 2.9 percent through January, limiting the first-class rate to an increase of just over a penny.

Under the new law, postal prices will be adjusted each May, the Postal Service said. Officials said they plan to give 90 days notice of future changes, twice what is required by law.

While 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 two-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 set for May 12:

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

— Money Orders up to $500, $1.05, unchanged.

— 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.

On the Net:

U.S. Postal Service: www.usps.com