It also depends on the Prohibited List of USPS, see
http://pe.usps.com/text/pub52/pub52c4_013.htm (I know Police is not a switchblade). I had USPS remove a ZT0350 w/o specifying any reason - the recipient received an empty box. Upon enquiry at his local PO, he was told their inbound international office received an empty box from overseas.