What is the difference between USPS Adult Signature and USPS Signature Confirmation?

The main difference is the required age of the signee. USPS Adult Signature requires the signing recipient to be at least 21 years of age, and can provide a government-issued photo ID to verify the signee’s age.

USPS Signature Confirmation only requires a signing recipient, regardless of how old they are. Technically, even infants can sign for packages, although that would be impossible because they lack the motor skills to sign stuff.