Gil has it right...it's a DOT requirement; seems that most companies charge about the same ($20-$25) per "box"; however, you can have up to 50lbs of powder per "box". If you were to order just one pound, it would be ridiculously expensive; however, if you have a large order, even with the S&H fees it ends up being cheaper. For example, a pound of Unique is going for around $25 in my neck of the woods. On a couple of websites, I've found it for around $18....so if I spread that hazmat fee over, say 20-25 lbs, I can reap a savings.
Hazmat fees are supposedly to help cover the "extra" cost of hazardous material, yet I don't see anyone approaching the door in a hazmat suit, ect....it still comes in a brown, cardboard box; the only difference is that it requires a signature.
....and I'm starting to thing that the whole thing is arbitrary. For example, I have to pay a hazmat fee for shipping primers, but not for primed shotshell hulls or for loaded ammuntition.....go figure!
