I got a ND "non-game" license, $17 online.
Form 6 is pretty easy, toughest part I found was figuring out exactly where to send it. Otherwise there is nothing out of the ordinary on it. I left the ammo portion empty, and it didn't cause any hassle. In regards to the invitation part, every if you are going to an event, get the hunting license. I know a few guys that only got their form 6 approved for the event the specified, rather than the full year like normal.
If you are taking any firearms you don't have the old registration certificates for, stop into Canadian customs on the way down. Get the green revisitation card. It will make life easier on the way back in.