Any extra ounce(s) are subject to a fee of $0.22 per added ounce as of May 31st, 2015 (previously $0.21 per oz). Another prior rise in rates occurred January 27, 2013 when the price of a first class letter sent through the United States Post Office went up $0.01 to $0.46.ome Ship & Postage. Note: the post office postponed their rate increase that was proposed to occur April 26, 2015. And the price of a first class (1) ounce letter sent through the United States Post Office remains at $0.49 for letters mailed within the United States.