I agree with
tiggerc that a SELECT statement should be used to return the data you want. Then you can keep it simple. I believe the code below does what you are looking for.
SELECT c.ID, Sum(t.Amount) AS SumOfAmount
FROM Customers c INNER JOIN Transactions t ON c.ID = t.CustID
WHERE (t.Confirmed)=True
GROUP BY c.ID;
Amount - Currency (Double)
Confirmed - Boolean