Solution:

Mike sends a box with a ring locked with the padlock.
Jane attaches her padlock to the box and sends it back to Mike.
Mike removes his padlock and sends the box back to Jane.
Jane removes the padlock and opens the box. She gets the ring and hopefully says "Yes".

Solution:

Mike sends a box with the ring locked with the padlock.
Jane attaches her padlock to the box and sends it back to Mike.
Mike removes his padlock and sends the box back to Jane.
Jane removes the padlock and opens the box. She gets the ring and hopefully says "Yes".