Stamps that can deliver letters to dead people and bring an answer back

This is a long shot because my recollection is rather vague, but around 40 years ago when I was a kid, I read a story about a boy who somehow acquired special stamps. If you addressed a letter to a dead person and put one of these stamps on it, the stamp would deliver the letter and bring a reply back. I believe he uses them to communicate with his dead mother. He might even be able to watch the letters float away to their destination. That is about all I remember about it. I'm not even completely sure it was stamps and I'm not totally sure it was a boy, there is a possibility it was a girl.