Subscribe to any SKU: back-in-stock, or price-drop below your number. Subscriptions store privately on the live host; the hourly sweep compares them against the live catalog and composes the email the moment the condition is true — once, never spam.
On the storefront this lives as a one-click button on the product page — this Labs page is the raw desk for testing it. Alerts fire through the same mailer as everything else: composed-and-logged until the email key is set, then delivered for real.