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..