﻿ The US Postal Board of Governors approves all USPS Postal Rates for sending USPS Certified Mail. The cost to send a 1-ounce USPS Certified Mail letter with Return Receipt (green card) is $6.74. The new Certified Mail rate became effective Sunday May 31, 2015.he cost to send a 1-ounce USPS Certified Mail letter with Return Receipt (green card) is $6.74. The new Certified Mail rate became effective Sunday May 31, 2015.