All mailable items that may be sent as Priority Mail International, except those items allowed in the Priority Mail International flat-rate envelope, may be sent in Priority Mail flat-rate boxes (see 231). The contents must fit securely and must be entirely confined within the box. The box flaps must be able to close within the prefabricated folds.
A flat-rate box may be insured. See 320 and Individual Country Listings for insurance availability, limitations, and coverage. Registered Mail service is not available.