I've sent serveral cabs in the post, though only to UK mainland, but as with all couriers, you have to pack for the lowest common denominator. In other words, those that dont care.. dropping it , etc.
For cardboard, and i mean that hefty stuff that barely bends, i use my local fridge / freezer / cooker shop, who throw their cardboard into a skip, and are happy to have some taken away, as they pay for removal, and anything taken means they get more in. I've used them umpteen times, and they also have some serious polystyrene slabs that the white goods arrive with, as well as that industrial cardboard.
So bubble wrap first, double cardboard on all corners and edges, then cardboard all round. Also, duct tape is better than packing tape.