PostgreSQL - Grouping all client subscriptions and multiply unitary value by quantity only when product ID is not 64

gistfile1.sql

SELECT
  client.id,
  client.name,
  fop.start_effective_date,
  fop.end_effective_date,
  EXTRACT(month FROM fop.end_effective_date)::integer AS month,
  SUM(CASE subscription.product_id WHEN 64 THEN subscription.unit_value ELSE subscription.unit_value * subscription.qty END) AS total
FROM
  master
LEFT JOIN
  subscription ON subscription.client_id = client.id
LEFT JOIN
  fop ON fop.client_id = client.id
WHERE
  EXTRACT(year FROM fop.end_effective_date) = 2011
GROUP BY
  client.id, client.nome, month, fop.start_effective_date, fop.end_effective_date
ORDER BY
  month