Can an invoice be edited or can a refund be issued after it has been marked as delivered?

Yes, there is no restriction on editing the invoice, issuing a refund, or creating a credit note based on its delivery status, whether it is “Delivered,” “Undelivered,” or “Partially Delivered.” You can click “Edit” and save the changes normally, as well as create a refund or credit note, regardless of the delivery status.

The delivery status is completely separate from these actions and is managed independently through its own delivery actions (“Mark as Delivered” / “Mark as Undelivered”) or by controlling individual items from the Stock tab.