How much does it cost to send a Certified Mail letter? ← Postage on a Certified Letter. How much does it cost to send a Certified Mail letter? $6.47 is the current USPS Certified Mail rate to send a 1-ounce Certified Mail letter with Return Receipt Requested service (old fashioned green card). Postal rates decreased the costs of the Certified Mail fee, First Class postage and the old fashioned green card. Follow this link to learn more about the USPS Certified Mail rates.