This error can most often take two forms:
SHIPMENT.POSTAGE.FAILURE: Unable to complete shipment purchase: Missing but required: customs contents type.
- SHIPMENT.CUSTOMS_INFO.REQUIRED: Unable to proceed, 'customs_info' is required for international shipments, shipments bound for US military bases, or US territories. Please see https://www.easypost.com/docs/api#customs for more information.
This happens if you are trying to generate an international label, or a label to a military base. We do not yet support automated international labels.
Note that we will support international labels though Shippo only (coming soon).