The SUM of the column should do it. Notice it is the first thing in the query -not because it has to be done like that but because I didn't want it to get lost in the sea of columns and tables in the query.
$sql = "SELECT SUM(order_details.UnitPrice) as TotalPrice, customers.CompanyName, orders.CustomerID, orders.OrderID, order_details.OrderID, order_details.ProductID, products.ProductID, products.ProductName, order_details.UnitPrice, order_details.Quantity, orders.shipVia, shippers.shipperId, shippers.CompanyName, orders.Freight
FROM customers, orders, order_details, products, shippers
WHERE customers.CustomerID = orders.CustomerID
AND orders.OrderID = '" . $_GET['order'] . "%'
AND orders.OrderID = order_details.OrderID
AND order_details.ProductID = products.productID
AND orders.ShipVia = shippers.ShipperID
ORDER BY products.productName, products.UnitsInStock, products.UnitPrice, shippers.CompanyName, orders.Freight";