I've been shipping commission models for 20+ years all over the world. Joberg is correct about "depends on the type of model" .
When I ship aircraft, there is landing gear, ailerons pointy things etc. etc. poking out everywhere. so i 'float' it in polyester fill (looks like clouds) then i put it in a box inside another box.
But if it is large and heavy, then I add bracing and strategically place tape. The larger the model, the LESS room for movement. Always allow 1 to 2 inches around the model for the shipping company's poor handling practices.
I always tell the recipient that 'things may break, have a bottle of super-glue to repair".
That's my 2 cents. !!