This is a self-educational project as I am very much a newbie when it comes to electronics, radio, etc. This also means that some of the ideas I have might not just be feasible due to the laws of physics I am not particularly aware of.
My current idea is to simply sense increased luminosity (somebody opened the mailbox) as a proxy for delivery and LoRa to send a notification of the event. I've seen a previous attempt to make a commercial tray project with a much more sophisticated approach and even a SIM card, but I think this is way too much. I wanted something simpler. And, perhaps, cheaper -- and no subscription.