Calculate the average salary per order for Andrew Fuller.
average salary = AVG(Salary)
SELECT CAST(SUM(T1.Salary) AS REAL) / COUNT(T2.EmployeeID) FROM Employees AS T1 INNER JOIN Orders AS T2 ON T1.EmployeeID = T2.EmployeeID WHERE T1.FirstName = 'Andrew' AND T1.LastName = 'Fuller'