:). OTOH, as someone interested in high latency communication protocols I wish cryptographers would hesitate to suggest a three round trip protocol at postal latency.
My solution is that Jan should send the ring in a padlocked wooden box with an inner protective but unlocked box and Maria should apply a saw to the outer box.
(To be fair, I can certainly imagine that in Kleptopia they know how to make excellent boxes).
Even simpler: send it in a padlocked cardboard box which Maria can rip open with her hands. Nothing in the problem statement says that the box itself has to be resistant to attack.
My solution is that Jan should send the ring in a padlocked wooden box with an inner protective but unlocked box and Maria should apply a saw to the outer box.
(To be fair, I can certainly imagine that in Kleptopia they know how to make excellent boxes).