WASHINGTON — Buy those Forever stamps now. The cost of mailing a letter is probably going up again.

Fighting to survive a deepening financial crisis, the Postal Service said yesterday that it wants to increase the price of first-class stamps by 2 cents, to 46 cents, starting in January. Other postage costs would rise as well. The agency’s persisting problem: ever-declining mail volume as people and businesses shift to the Internet and the declining economy reduces advertising mail.

“The Postal Service faces a serious risk of financial insolvency,’’ said Stephen M. Kearney, postal vice president, an indication that without significant changes, the agency would eventually be unable to pay its bills.

The post office lost $3.8 billion last year, despite cutting 40,000 full-time positions and making other reductions, and Kearney said it is facing a $7 billion loss for this year and the same for fiscal 2011, which begins in October. The rate increase would bring in $2.5 billion, meaning there still would be a large loss for next year.

The post office, though part of the federal government, does not receive a tax subsidy for its operations.

People who bought Forever stamps at the current 44 cents or at lower prices would still be able to use them without paying the difference.

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