Upload customs-related documents (such as a commercial invoice) to an existing international shipment so the carrier can process customs clearance.

When to use: Call this endpoint after generating a label and a commercial invoice for an international shipment. Some carriers require documents to be uploaded separately before the package can clear customs.

Prerequisites:

• Valid JWT authentication token (Bearer)
• A carrier ID and tracking number from a previously generated international shipment
• One or more document URLs (e.g., the commercial invoice PDF returned by /ship/commercial-invoice)

Key behaviors:

• Accepts documents as URLs pointing to PDF files
• Supported carriers include FedEx, DHL, and UPS (carrier support may vary by origin country)
• Returns a confirmation indicating whether the documents were successfully attached to the shipment
• Documents must be uploaded before the carrier's cutoff time for the shipment to avoid delays