This new rate would be $0.48, one cent below the Post Office or retail rate of $0.49 for First-Class 1 oz. Letters.• First-Class Mail Flats/Large Envelopes (1 oz.) rates will increase six cents from $0.92 to $0.98. Each additional ounce will cost an extra $0.21 (one cent increase from 2013).• Postcard rates will increase one cent from $0.33 to $0.34.