Its too late for him to put in a claim. He didn't recently recieve the transmission. He's messing with you. No one waits a month or so for something they paid that type of money for. Within a week I would of contacted the seller if I didn't recieve my item. Point is, he got it within a week or two after you sent it, no later then that. It was installed and he beat the shit out of the transmission. Its highly unlikely that 2 gears 3rd and reverse would suffer damage during shipping.

Just don't worry about it. Its his problem now.