The U.S. Postal Service has announced a postage rate increase for select mail classes and services for 2015. The new postage rates will start on Sunday, April 26, 2015. First Class Letters & Postcards. The cost to send a one ounce First Class Letter will NOT BE INCREASING and will remain $0.49.Additional ounces will cost $0.22, a one cent increase from 2014. Postcard rates will increase by one cent to $0.35, from $0.34 in 2014.he U.S. Postal Service has announced a postage rate increase for select mail classes and services for 2015. The new postage rates will start on Sunday, April 26, 2015. First Class Letters & Postcards. The cost to send a one ounce First Class Letter will NOT BE INCREASING and will remain $0.49.