Hi Lynne,
In theory this sounds like a great idea, but in the US it is illegal to put anything in a postal mail box (it must have postage affixed - mailed and delivered by the post office). You might check with the laws where you are as to whether it is okay or not.
In the US it is okay to put in a door slot though, because it isn't a postal box.
Sorry to be a wet blanket. Just check with where you live first (Canada I think).