Mailing an envelope with two pieces of ordinary stationery via first-class mail within the United States costs 49 cents, as of June 2014. This standard rate is applicable to any envelope that weighs 1 ounce or less. It costs $1.15 to mail the same letter internationally, according to Stamps.com.
Letters that weigh more than 1 ounce receive an additional charge of 21 cents per each ounce over the first ounce. It is possible for an envelope containing two pieces of paper to weigh more than 1 ounce if the paper is unusually heavy, or if the envelope is oversized. It's prudent to weigh the envelope prior to mailing to confirm the correct postage.