project lazy shelf

Hello Arduino comunity.

I'm fairly new to arduino and the whole process around programming arduino.
Im currently doing a project at college for my teacher.

The project is: Automation of a mail shelf

my thoughts is that an arduino uno and a Ethernet shield that with either infrared or proximity sensor
can determine weather or not my teacher has gotten mail in his shelf and then send him a email telling him to go and get it.

Seeing how i'm not much of a programmer i've been playing around with a arduino mega ADK just to get a feel for it, and the basic
things of getting leds to light up and turn them of with switches. If you have any good thoughts on what kind of sensor i might need
feel free to comment. Any tips are greatly appreciated

Perhaps a light sensor in the shelf detecting when it is obscured by a letter? I'm visualising an Arduino in a thin package which can be sat on the shelf so that letters are normally placed on top of it.

How about an IR transmitter/receiver combo?

Good description here:

You could mount the receiver on the bottom of the box, and flood the box with IR (eg. from an Arduino on its side on the wall or at the back). The sketch could check if the IR gets through (in which case the receiver is not covered). Using encoded IR means that ambient light shouldn't affect it.