Registered mail is an add-on service offered by USPS that provides extra protection for high-value letters and packages. When you send a letter by Registered Mail, the USPS establishes a chain of custody that tracks and secures your shipment throughout the entire transit process—from the moment it is dropped off at the Post Office until the moment it’s delivered.
You send letters as Registered mail through the Lob API by passing the value registered in the “extra_service” parameter of the create letter request.
Please note that the Registered mail add-on is only available for First Class mail, and can be sent both domestically and internationally.