Are shipments of letters and documents transported by air otherwise than through the USPS subject to the advance cargo information requirements? Is there a weight limit for a shipment to qualify as a letter or document?

Are shipments of letters and documents transported by air otherwise than through the USPS subject to the advance cargo information requirements? Is there a weight limit for a shipment to qualify as a letter or document?

 The Federal Register Notice published on December 5, 2003, also noted that letters and documents, including the material described in General Headnote 19(c) of the Harmonized Tariff Schedule of the United States would be the subject of a separate publication in the Federal Register.  CBP has not established a weight limit for letters and documents for purposes of this rule.