In the last 2-4 years, the max/limit size the US postal service can send appears to have shrunk - based on posting stuff back from botcon each year.
As such, I wouldn't be surprised if the size of Fort Max (in a box big enough for it) is now too big to send by USPS. If that is true, and the seller is willing, see if they can find out if UPS (a courier company) can handle it.