Of all the shipments made by United Package throughout the year 1996, what percentage correspond to the month of September?
DIVIDE(COUNT(OrderID where CompanyName = 'United Package' and ShippedDate > = '1996-09-01 00:00:00' AND ShippedDate < '1996-09-30 00:00:00')), (COUNT(OrderID where CompanyName = 'United Package' and ShippedDate > = '1996-01-01 00:00:00' AND ShippedDate < '1997-01-01 00:00:00')) as percentage; 

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%'