International Mail (CBP Shipment Type)
To qualify the shipment must be moving directly from the foreign postal service, e.g. Canada Post, to the US Postal Service, and must consist only of letter class mail or correspondence. Merchandise would require a formal or informal entry and would clear under other shipment types.
International Mail and ACE eManifest
In ACE eManifest there is presently no functionality to declare an International Mail Shipment. As such, highway carriers can simply leave the International Mail Shipment off when preparing their manifest, and have the driver verbally declare the shipment and present the paperwork to the officer.