You want to send a valuable gift to a friend and you want to put a lock on it so nothing happens. Your friend doesn't have a copy of the key and you need your lock back.
How do you do it?

You send it with your lock, once they get it, they send it back with one of their locks. When YOU get it back, take YOUR lock off and send it back. They have the key to their lock which should be on it, and remove it.... there is their valuable gift.