Try the Monoprice 24259 mailbox sensor, on the first post you can find a link to the sensor. It’s designed specifically to work with mailboxes, it’s got a good range and runs on AA batteries, it also has tamper detection if anyone tries to remove it.
To help it out, add a simple Z-Wave Plus mains powered Switch/Outlet/Plugin outside your house in direct line of sight with the sensor. This will go a long way in improve the mesh/communication with the sensor. (like the outdoor GFCI outlet or the the cloest indoor outlet you can find)
This is what it looks like:
