So how do you get something as big and awkward as a bicycle into a box small enough to meet the strict USPS overseas shipping requirements, and ensure its safe arrival? It helps to have a lot of experience, a good box to start with and some basic math skills to optimize the box size.
It doesn't seem possible to shoehorn that bike fit into that box, but time and time again it works...
It may be a lot of work, and it may take a long time but in the end the feeling that your prized ride will arrive at its destination safe and sound and at a reduced price is worth it. I usually remove the fork, bar/stem, cranks and remove the rear derailleur and chain, bag them and tape to frame behind the BB.
In the ten plus years that I've been involved with shipping bikes across the world I have not had a single one damaged and have never had to file a single claim.