Matt Williamson wrote:

I'd use a spring switch with a top plate that covers the bottom of the
mailbox. Make sure the spring is strong enough to hold the plate up
but any additonal weight will trip it. Use a battery to light the
led, put the switch in between the led and battery. When there is
mail, it will trip the switch and light the LED. When you remove it,
the spring will will push the plate up and it will go off. You'll
probably need a resistor between the battery and LED depending on
what type of LED you use.

Ohhh! That triggered an idea!

Get one of those spring snakes you find in trick boxes of peanuts and jam it
in the box.

When the (presumably) female mailman opens the box, the high-pitched scream
will alert you to a delivery.