Puzzles/How do you ... ?/Sending a Package/Solution

Dmitri secures the package with a padlock and chain and sends it to Ana. Ana attaches another padlock to the package and locks it. She then sends the package back to Dmitri. Dmitri removes his padlock and sends the package back to Ana. Ana removes her padlock and opens the package. alternative: Dmitri sends the locked package to Ana while also sending a separate letter with the key a few days later. Or if a combination lock, Dmitri sends a letter a few days later with the code. problem with above: the unlocked letter with the key may be opened and the key stolen..