What the PIR sensors measure is a very small change in heat moving across the field of detection. So there is a motion component. Positioning the motion sensor so it’s facing straight out to the mailbox opening is the worst position for it, though. It’s much better if it’s on the side or the top of the mailbox facing down so that the act of putting mail into the box will cross the plane of the sensor.
Another popular option is to put a tilt sensor on the mailbox door. Again, the transmission will be sent when the door is open.
Many people do use motion sensors or tilt sensors with mailboxes, the trick is just getting the Z wave signal or zigbee signal out of the box and back to the house. But if the plan is just to send a signal when the mailbox door is open, it may work just fine depending on the exact angles and distance.
If your mailbox is metal, you’re going to have a lot of issues with the contact sensor because the metal around the magnet part will magnetize overtime and that confuses the sensor. It would probably start reporting that it was always closed. So i’m not seeing the sensative strips as a good match for a metal mailbox application.
My own preference would likely be for a tilt sensor on the door first, as that’s just simpler. After that, a motion sensor just inside the box facing across.
Z wave plus has a longer range, but zigbee travels somewhat better through rain.