The cost of a first-class U.S. stamp just went up. The rate change went into effect Sunday.Metered one-ounce letters are increased by a penny from 63-cents to 64-cents.The cost of sending a postcard domestically increased from 51-cents to 53-cents.Mailing international postcards or one-ounce letters overseas increased from $1.50 to $1.55.Forever stamps, first issued in 2007, remain valid for full First Class Mail postage no matter when the price increases after their purchase.It’s the Postal System’s third rate hike in 12 months, and the 18th since 2000. The cost of a stamp last went up last July when a oneounce letter ...