Has anyone actually posted a key to another country?

My keyholder is about a thousand miles away and has floated the idea of me posting her the key. She likes that it would be a real object in her house rather than a symbolic arrangement where I have the key in a drawer and promise not to touch it.

I like the idea too. What I keep getting stuck on is the practical side. If I post it and it vanishes, then what. And is there any point when I would still have a backup key in the house, which I am not giving up.