Just use the summary function in a HAVING clause
SELECT SUM(Invoice.TotalPrice) AS "Total", Customer.LastName, Customer.FirstName
FROM Invoice
INNER JOIN Customer
ON Invoice.CustomerID = Customer.CustomerID
WHERE DATEDIFF ( MONTH , Invoice.DateOrderPlaced , GETDATE() ) > 3
GROUP BY Invoice.CustomerID, Customer.LastName, Customer.FirstName
HAVING SUM(Invoice.TotalPrice) > 20000