You can lose one cross apply and further simplify the query like this:

SELECT

o.value('(del_Nr/text())[1]', 'VARCHAR(10)') as PONbr,

o.value('(del_Third_Nr/text())[1]', 'VARCHAR(10)') AS NCubeID,

o.value('(Line/Product_Code/text())[1]', 'VARCHAR(30)') AS InvtID,

o.value('(Line/Product_Quantity/text())[1]', 'INT') AS QtyOrd

FROM #xNESP_XmlImportPOData

CROSS APPLY xml_data.nodes('/message/body/shipmentorder/orderheader/data') a(o);