Providing your logistics partner TNT the IOSS number correctly ensures your shipments are handled appropriately.
Marketplace IOSS numbers need to be mapped to an Order Extended Property called SenderVATNumber, as TNT does not have a separate field for IOSS numbers. They will automatically be passed to the courier if present on the order.
Shipper IOSS can be added in the TNT Config under the VAT No field. This value will always be passed to the courier if there is no Order Extended Property added with the SenderVATNumber name.
Ensure that the Incoterm (Shipping > Integrations > Services > Edit) has the value of Delivery Duty Paid (DDP).