Compute the total payment made by Sarah Lewis for film rentals so far.
total payment = sum(amount)
SELECT SUM(T3.amount) FROM rental AS T1 INNER JOIN customer AS T2 ON T1.customer_id = T2.customer_id INNER JOIN payment AS T3 ON T1.rental_id = T3.rental_id WHERE T2.first_name = 'SARAH' AND T2.last_name = 'LEWIS'