Is Registered Mail a separate class from Priority Mail?
No. Registered Mail is an add-on service that rides on top of a mail class, usually Priority Mail, First-Class Mail or USPS Ground Advantage. There is no such thing as sending something Registered Mail on its own; the real choice is Priority Mail by itself, or Priority Mail with registration added.
How much does adding Registered Mail to a Priority Mail package cost?
Registered Mail starts at 20.90 USD with no declared value, per usps.com, on top of whatever Priority Mail postage the box already costs. The fee climbs from there in bands tied to the value declared at the counter, separate from the postage underneath it.
Which one arrives faster?
Priority Mail, and it is not close. USPS publishes a 2-3 Days estimate for Priority Mail. Registered Mail carries no published day count at all, because every handoff is signed for and the piece is held apart from ordinary mail, which adds time at each step rather than saving it.
How much coverage does each one actually carry?
Priority Mail includes up to 100 USD automatically and can add declared-value coverage up to a 5,000 USD ceiling. Registered Mail's own maximum indemnity is far higher: up to 50,000 USD per piece, per usps.com, which is the main reason anyone chooses registration over ordinary coverage.
Can I register a padded envelope?
No. DMM 503.2.1.1 states registration cannot be obtained on a padded envelope, a Tyvek or plastic mailer, glossy-coated paper, anything tied or fastened with articles, or business reply mail. Registration is a counter transaction on a plain paper envelope or a box, not something added to just any package.