Database: retail_world
Question: Of all the shipments made by United Package throughout the year 1996, what percentage correspond to the month of September?
SQL: SELECT CAST(COUNT(CASE WHEN T1.ShippedDate LIKE '1996-09%' THEN 1 ELSE NULL END) AS REAL) * 100 / COUNT(T1.ShipVia) FROM Orders AS T1 INNER JOIN Shippers AS T2 ON T1.ShipVia = T2.ShipperID WHERE T2.CompanyName = 'United Package' AND T1.ShippedDate LIKE '1996%'