The shippers may or may not always honor what the law states. Some just don't want to ship for non-FFL's even in those situations where its legal. The mom & pop shippers aren't experts on the law, so its easy for them to say, no. Going thru a dealer is the safe way. The dealers (both of them) will however have to log the gun in and out of their books.
btw, the law does say that the receiver's ffl must be in the box. An ffl isn't required to ship a gun, only to receive one. (Gunsmiths only can ship back to the origonal shipper).
Your son in Michigan, shipping to you in another state might be ok if there is a bill of sale in your name with the gun. I'd have to get a ruling from BATF in writing on that though, as some of their agents interpret the law in ways that might not hold up in court. But its not worth $50 to find out. You did the easy thing.