You mean a reed switch?
You could use that but typically it's a bit bulky, whereas this TINY hall sensor is vritually non existent next to the Moteino which itself is very small.
You could also use a small tilt switch where the internal mercury or metal ball moves when there is movement causing a contact closure. But those are mechanical so prone to error sometimes.
I am experimenting with a PIR sensor with very good results. I may release a kit based on this method. The PIR consumes only 60uA while non active and about 260uA when motion detected, causing an interrupt which can wake the Moteino on pin D3.
So yeah I would recommend trying PIR. That one does not depend on a magnet or another part that it has to align with. It will always detect the motion when the box opens.
The hall effect uses 2.5mA according to DS:
http://www.melexis.com/prodfiles/0004822_US5781_rev014.pdfSo leaving it ON is a huge energy loss.
Also 1 second mailbox open is about the max I would allow for security purposes, someone could peek in your mailbox for less than 1 second, so you may want to catch that action. With the PIR you don't have to worry about that. Any motion will trigger it.