Email me when a flight fare drops below my target

Once a day, your fare provider's search endpoint is asked what one route costs on the date you want to fly, and the cheapest offer that comes back is compared with the price you would actually book at.

You are emailed on the change, not the state. The first day the fare sits at or below your target you get one alert with the price, plus the airline and a booking link where the provider supplies them. While it stays there you hear nothing more, and when it climbs back above your target you get a single note saying the watch is armed again. It knows what it has already told you by searching your own mailbox for the newest alert about this route, so nothing is repeated. If that search fails, nothing is sent rather than risk a duplicate, and once your departure date has passed the checks stop on their own.

You need a fare search endpoint and key from a travel data provider, answering with a list of offers that each carry a price.