Sending a document with two padlocks and an insecure courier

You want to send a document to a colleague through a courier who steals anything in an unlocked box (including any locks or keys inside). You and your colleague each own a padlock whose only key stays with its owner, and the box can carry several padlocks. How can the document arrive safely?

Use three trips with two locks. Lock the box with your padlock and send it. Your colleague adds his own padlock and sends it back. You remove your lock and send it again; it now carries only his lock, which he opens. The box is never in transit unlocked.