Griffin seems to believe that the post is an intruder and has shredded it on several occassions. The solution for us was a wall mounted letter box outside the front door.
This has helped but when we bring it in he still goes nuts for it. Or if we leave him in the kitchen and bring the post in to the lounge, he goes for it as soon as we let him out.
All I can think is he can smell it!
So, what we do now is bring the post in and I sit and hold it, make him wait then say ok. I let him sniff it all over but as soon as he involves teeth or paws (he likes to scrabble at it with his feet) we say "ah ah!!!" and stand up and only let him sniff again when he's calm. It hasn't solved it but at least he doesn't shred it now. Good luck!