1 The cost to mail a first class letter is 46 cents for up to one ounce, and each additional ounce adds 20 cents. 2  Large envelopes (flats) that are rectangular and not too rigid or thick are 90 cents for up to one ounce (and 20 cents for each additional ounce).3  Expect to pay 33 cents to mail a postcard.he United States Postal Service increased its rates for first class postage and related services (like insurance) on Jan. 27, 2013. The price of a first class Forever stamp increased to 46 cents from 45 cents.