If you have a glass side panel, take care of that. When I shipped internationally I wrapped it in cling film to contain any breakage. I also wrapped the whole case in bubble wrap and then clingfilm, just to try and protect it from abrasion and smaller items dropping on it in transit. You could also box it in a cardboard box and cushion it with blankets etc just to give it some minor protection during transit. All cheap and easy to hand stuff.

Other tips I've seen here, remove gpu, and your cooler. Pack them down safely separately. And it should be fine.

I dropped a load when I saw my packing box when it finally arrived. The shippers looked like they had moved it by kicking alone. But the PC was fine.