This must be nullable, as it comes back null for USPS manifest documents from Endicia.

kendallb · kendallb · commit e6e4a548daf6 · 2021-07-07T17:43:06.000-07:00
diff --git a/openapi.yaml b/openapi.yaml
@@ -8443,6 +8443,7 @@ components:
             - $ref: '#/components/schemas/se_id'
           description: A string that uniquely identifies the warehouse
         submission_id:
+          nullable: true
           readOnly: true
           type: string
           minLength: 1
