https://serverfault.com/questions/204361...ail-is-received

The replies to that thread claim that you can run an android app called Wake Me Up Smart Alarm which triggers an alarm when a specific email is received.

This isn't the same thing as the Alexa thing you're looking for, but it's getting into that neighborhood. For the Alexa doohickey, you're likely going to need to interface with the Alexa API, or convince the makers of the "Here Comes The Bus" app to do it.