Pyrodex doesn't fall under the blackpowder regulations.
From teh back of Pyrodex and 777 1lb bottle:
(Replace Triple Seven with Pyrodex, as appropiate) "Triple Seven is nto an explosive regulated by Federal Explosive Law, but may explode if misused."

Triple Seven and Pyrodex can be shipped with any standard shipping method, there is no limit to the quantities you are allowed to carry. I bought ~250 lbs from WalMart when they put it on clearance at $1.00 per lb. (Pyrodex and Triple Seven powder)