Mail detection

Hey,
I am planning my first bigger project that should really do something and I chose to do a sensor to my mailbox that would notify me whenever a mail gets dropped in.
I have the logistics of notifications all sorted out, I have my NodeMCU v3 - ESP8266 with wifi capability up and running and now I am thinking how to actually detect the mail.
The biggest problem is that I do not have a standard mailbox, but I have a euro style mailbox (see attachment) - it has “inside door” that isn’t closed all the way so you can throw a letter in without really moving it.
First, I was thinking about putting standard magnetic sensor, but those have usually around 0.5cm (1/5inch) wiggle room, so I don’t believe simple letter would trigger it.
I came up with IR gate that would detect break in line of sigh of the diode, but this plan failed at power consumption - I need to fit it reliably in the mailbox with a battery. And a gate like this would not last very long.

Do you have ANY idea or pointers where should I look?
Than you

mailbox.jpg

How long does it take the IR sensor to start up and make a single detection? The light only takes a few femtoseconds to cover the distance required.

How long does a letter occupy the detection zone when it's being pushed in by hand? Maybe one second?

So you could run the IR detector for a millisecond maybe twice a second. That's a power saving of 500 times less than running it full time.

Maybe a flat sheet of something, sized slightly smaller than the receptacle so any envelope will rest on it. When depressed by the weight of an envelope a microswitch or hall switch is actuated. With light enough springs a single envelope should actuate the switch.

Think spacebar.

MorganS:
How long does it take the IR sensor to start up and make a single detection? The light only takes a few femtoseconds to cover the distance required.

How long does a letter occupy the detection zone when it's being pushed in by hand? Maybe one second?

So you could run the IR detector for a millisecond maybe twice a second. That's a power saving of 500 times less than running it full time.

That is so true! and even more, I can put the sensor on the bottom, so I can fire it up every 5 minutes - that granularity is enough for me to know something is is.
This solves it, thanks for breaking me out of my box! that's what I needed