1. The model should be in its own box which is then placed inside a shipping box.
2. It is most critical that the model be secured within its own box so that it CANNOT move about. Simple physics dictates that if a moveable object hits a harder object it will get damaged. So if the model can move about within its own box, it will ultimately strike the sides and get damaged or destroyed. And that also means that if any parts can come loose on their own, and bounce around, they will damage the more fragile parts of the model.
3. It was suggested that the model be tied down using string, etc. This can work, but a word of caution: if the string is too tight, it can chafe against the sides or parts of the model, and damage or indent it. If not tight enough, the model may move and in so doing, the string or binding will tear off parts. It will work better of you can put foam between the model and the binding. But either way, itís not perfect. A better way may be to fashion cardboard partitions, but again, any chance of movement is a danger.
4. Once the model is totally immobilized within its own box, then it goes into the shipping box. The packing in the shipping box should be so full, and tight, that the model box cannot move. The basic principle is to secure everything, so that nothing can move within.
I hope this helps.