I have completed a sales order and as expected one line was added on completion for packing material. Now I expect this extra order line to have an invoice candidate that I can use to invoice the packing material but no invoice candidate exists for that line.
The invoice candidates for packing materials are not generated from sales order line but from picking. So they will appear once you picked the sales order lines and select the final and actual packing material there. This is to support a different packing material on shipping. Once the inout is created the packing material will have invoice candidate in invoice dispo.