While less automatic than your idea it might be more practical to just have rainmeter display the last date and time that the box was opened. From that you should quite easily be able to tell if it was you or the mail service.
Based on this idea, if you have a relatively small window of time each day in which the postal service normally delivers then a bit of logic could be employed to make a guess at who was accessing it.
A purely ping pong solution could also work using a variable but this would probably need user intervention to correct the status on a regular basis.