Podría colocar otra consulta alrededor de su original para producir la suma:
SELECT x.cuInvoiceID, x.PONumber, x.DueDate, sum(x.TotalPrice), x.trackingNumber, x.Year, x.isTaxPaid
FROM (<YourOriginalQuery>) x
GROUP BY x.cuInvoiceID, x.PONumber, x.DueDate, x.trackingNumber, x.Year, x.isTaxPaid