How can Tom send Sally the ring without it getting stolen?

Question

Tom wants to send Sally an expensive ring through the mail because they live thousands of kms apart. The only problem is that everything sent through the mail is stolen unless there is a lock put on it. Both Tom and Sally have plenty of locks and keys but they don't have any of the same locks or keys.

How can Tom send Sally the ring without it getting stolen?

Answer 1

Tom shall send the one set of key to Sally, Once the key is reached Tom shall lock the mail with the other set of key from the same pair and can send safely.