To mail a large envelope, you can use First-Class Mail® service. The postage for a large envelope (or flat) starts at **$1.26** for 1 oz . If the size of a large envelope is below or up to 1 ounce i.e 28.35 Grams, then you will have to use two Forever Stamps. As the cost of one Forever Stamp is $0.63, so two Forever Stamps will cost you $1.26 which will be the price of the Postage of 1 Ounce Large Envelope.