Answer
This has been confirmed not achievable — attempting to trigger an automatic attachment print this way (including trying to invoke the underlying print-agent function directly) doesn't actually reach the Aurena Agent the way a manual print does, so there's no working automation path for this today. The practical fallback is to have the workflow or report rule raise a user-facing alert reminding the person printing the delivery note that attachments still need to be printed manually, rather than trying to automate the attachment print itself.