It pays to check those musty old boxes in your attic.

An unidentified family in the Deep South made the discovery of a lifetime when they found a letter written by Founding Father Thomas Jefferson in which the third president extols the virtues of American independence and hails victory in the War of 1812.

“As in the Revolutionary War, [the British] conquests were never more than of the spot on which their army stood, never extended beyond the range of their cannon shot,” Jefferson wrote in the letter, penned at his Monticello home on Valentine's Day, 1815. "We owe to their past follies and wrong the incalculable advantage of being made independent of them. . . ”

“We must sacrifice the last dollar and drop of blood to rid us of that badge of slavery...”

- Thomas Jefferson

The letter, a response to one from U.S. Ambassador to France William Crawford, was found in a box tucked away in the family's attic among other heirlooms. The Raab Collection, a Philadelphia dealer of historical documents, is selling the letter on behalf of the family. The asking price is $325,000.

“This kind of letter is only seen up for sale once a decade, if not once a generation,” Nathan Raab told FoxNews.com. “You just never see this for purchase by the public. These types of letters that are owned by direct descendants are usually donated to private collections.”

The wide-ranging, four-page letter puts into sharp relief Jefferson's well-chronicled dislike of the British.

“We must sacrifice the last dollar and drop of blood to rid us of that badge of slavery, and it must rest with England alone to say whether it is worth eternal war, for eternal it must be if she holds to the wrong.”

Jefferson noted future president Gen. Andrew Jackson’s seminal victory at the Battle of New Orleans -- the final battle of the War of 1812 -- that led to America’s victory.

“It proved. . . that New Orleans can be defended both by land & water; that the Western country will fly to its relief . . . that our militias are heroes when they have heroes to lead them on,” he wrote.

Jefferson also comments on Napoleon’s demise and how it eventually worked to America’s advantage.

“[His] downfall was illy timed for us,” he said. “It gave to England an opportunity to turn full handed on us, when we were unprepared. No matter. We can beat her on our own soil . . .”

Rabb told FoxNews.com the letter is a rare treat for historical buffs and others.

“It’s a powerful and evocative reminder of our Founding Fathers,” he said. “To read about the country’s independence from the pen of Thomas Jefferson is incredible.”

The letter was put up for sale on Monday -- exactly 190 years after Jefferson's death on July 4, 1826.

Perry Chiaramonte is a reporter for FoxNews.com. Follow him on Twitter at @perrych