I’ve been thinking of doing something similar just for the fun of it. Yes, it’ll need wifi, if on wifi, should be pretty easy to use IFTTT to send you a notification. You could certainly do it with a Boron for pretty quick/easy but $36/year and that does require more power even if connecting once a day for a mailbox notification system might be more than you want to spend and you’d have to occasionally recharge the battery.
If I ever get around to it, I’d look at doing it with LoRa or LoRaWAN. That certainly has the necessary range and could last years on battery. Downside is it would require a LoRaWAN gateway or LoRa receiver. I’m using LoRa paired to a particle device to essentially have Particle act as a Psuedo private gateway for remote LoRa nodes. This is partly why I also tend to think LoRa might be a good solution for mail box alerts. Just depends how much work you want to bit off. If you have wifi, go with Photon or Argon, if you don’t have wifi, then yeah boron would be the easiest with some downsides of power and $3/month, if you want to spend a little more time developing, go with LoRa remote note to a Particle device with a LoRa radio, if you have a lot of devices and other ideas like this, maybe consider LoRaWAN or the LoRa to Particle I mentioned.
Sounds like a fun project!