It's a common scam-
He sends you a money order, including some in excess, you 'cash it', send him the remainder and your item. Two weeks later, the check is bad.
However, your version is different... I'd play along and tell him 'just write your check to the shipping company and they can pay me cash when they pick it up.'
For great laughs, check out http://www.419eater.com/html/trophy_room.htm
And the best revenge story ever on a scammer: