Our bog-standard postbox is about 50m from our house and we do not normally see our postie from the house. So to check whether we have mail I go to the box, lift the flap and peer inside. Always awkward, even after I glued on the inside floor of the box some tape, enabling me to spot something easier.
Today I thought 'I’ll drill a 25mm hole in the side - that will give me better view;. So I did - happy.
My neighbour saw me doing this and said - ‘there is an easier way’. Apparently you can get a balance that you place inside the box and ‘zero’ it. There is an app (free, he says) so you just point phone at box and it says whether or not you have mail (which will have upset the zero).
Anyone know of this? Or anyone use the US system of a little arm on top of the box that the postie moves to indicate he’s put something in?
Do the scales in question integrate some kind of wireless communications module (Bluetooth, NFC, or some such) ?
Otherwise, the only thing I can think of with electronic scales is the presence of a magnet and magnetometer/ Hall effect sensor, which the app on the smartphone can detect somehow, although I would have thought that distance from phone to scales would be an issue.
Good exercise.
FWIW, there are already systems to indicate whether “You’ve got mail !”
https://www.boite-aux-lettres-normalisee.com/241-boite-aux-lettres-avec-avertisseur-de-courrier.html
however, that does require you replacing your mail box 
Wouldn’t be much use to me, not having a smartphone, but, as we also can’t see the post box from the house, many years ago I connected a bell (brass with a clapper inside) inside the wooden framed porch to our kitchen door via a length of rope through all sorts of little pulleys, to the frame that the boite was mounted on. On the end of the rope was a wooden knob which had outlived its previous life on the end of a wooden curtain rail.
The postman happily pulled this knob for years until the scrapping of the wood framed porch and its replacement by a double glazed PVC framed veranda made drilling a hole for the rope undesirable.
Now, I take the dog for a walk every day to see if there’s anything in it. 
Place a false floor on the box on a fairly sensitive spring. Drill a hole in the base of the box and dangle a weight, with the bit of string inside the box coloured. If the weight is lower and you can see the coloured bit of string you have mail…
Or just ask your postie if they would raise a flag
I think that is a great idea now that you can put letters in for them to take, it has bugged me that if I wasn’t getting mail how would they know to stop to pick up a letter.
I would like a different coloured flag though
entirely by chance if they are passing but here in the Charente, the mairie will provide you with a magnetic token - Postale sticker which you put on the front of your box to alert them (and after they’ve taken the mail, they place the sticker back in the box) although the other accepted way is to clip a clothes peg to the flap (whatever colour you like!)
I know this, because on one occasion I had a bright idea 
In order to establish if the post had been, I stuck a peg on the flap thinking that the postie would stick it in the box after placing any mail… oh no! she thought we had some mail to give her or were in some distress so knocked on the door to enquire… she explained, kindly, the routine. 
les Anglais 