This is a simple two transistor latching switch that turns on a 555 timer circuit to flash an led.  The entire circuit including the battery pack fits perfectly inside an Altoids tin.  I mounted the tin in the back of the mailbox with the reset switch and led mounted thru the back of the mailbox.  The triger wires were siliconed to the top inside of the mailbox with the trigger switch attached to the inside front against the mailbox door.  When the door is closed, the door pushes against the lever on the switch and opens the trigger switch.  When the door is opened, the switch closes and trigers the circuit.  The circuit can be trigered over and over without reseting the circuit.  With the door closed and the reset switch puished, the circuit returns to the standby state.  The current draw in the standby state is only a few microamps so battery life will be long. It's cheap, simple and could serve many purposes. Attached are pics of project, circuit and parts list.