By using CROSS APPLY with a single subquery that returns the necessary columns, I can cut down the number of logical reads and the number of touches on the Sales.SalesOrderDetail table.

what-is-sql-cross-apply-10.sql

SELECT
    SalesOrderDetailID,
    UnitPrice,
    CarrierTrackingNumber, x.*
FROM Sales.SalesOrderDetail o
CROSS APPLY
(
    SELECT
        RunningTotal = SUM (UnitPrice),
        RowCnt = COUNT (*)
    FROM Sales.SalesOrderDetail i
    WHERE i.CarrierTrackingNumber = o.CarrierTrackingNumber
        AND i.SalesOrderDetailID <= o.SalesOrderDetailID
) x
WHERE o.CarrierTrackingNumber IS NOT NULL
ORDER BY CarrierTrackingNumber ASC;