The cost of USPS (United States Postal Service) stamps goes up all the time, in order to keep up with inflation. The chart above shows the price of a stamp in January of each year going back to 1913 (red), as well as the actual value (in 2010 dollars) for each year after adjusting for inflation (blue).
Prices before 2007 are equal to the price charged for each ounce of an item's weight, each ounce requiring one stamp. After 2007, the price of the first ounce and each additional ounce would become separate.
Source: http://www.usinflationcalculator.com/
