Postage stamps in the United States are valid in perpetuity. A 39-cent stamp will always be worth 39 cents. Because the current price for mailing a 1 ounce first class letter is 55 cents, any combination of stamps totaling 55 cents or more is enough. However, if the cost of sending a letter increases to 65 cents or more, then a 1 ounce first class letter must include a $1 stamp.