How many forever stamps to mail a 9 by 12 envelope?

To mail a 9 by 12 envelope within the United States, you typically need two Forever stamps, as the cost for mailing a large envelope (flats) is higher than a standard letter. As of my last update, the rate for a first-class large envelope starts at the price of a single Forever stamp, and additional postage is required for extra weight. Always check the current USPS rates, as they may change.