With the exception of the flat-rate envelope, Priority Mail International is a parcel service. Written correspondence having the nature of current and personal correspondence is not permitted in Priority Mail International parcels, but may be sent in the Priority Mail International flat-rate envelope.
All items that may be sent as First-Class Mail International (see 241) may be sent in the Priority Mail International flat-rate envelope provided that the contents are mailable and fit securely in the envelope and the contents are entirely confined within the envelope with the adhesive provided as the means of closure. The envelope flaps must be able to close within the normal folds. Tape may be applied to the flap and seams for closure or to reinforce the envelope, provided the design of the envelope is not enlarged by opening the sides of the envelope and taping or reconstructing the envelope in any way. Registered mail service is available. Insurance is not available.