Marks a manifest import complete after it has been marked as AutoImport
ManifestID - The manifest which is to be mark as complete.
UserID - The UserID (from List Users) who is marking the manifest as complete. Optional only if the Account only has one user.
A data structure like so:
{ 'result': [ { 'success': true } ], 'generated_in': '268ms' }
https://api.fastway.org/v3/fastlabel/markmanifestcomplete?api_key=YOUR_API_KEY