When I’ve ordered primers and/or powder from other vendors (ex. Midway) I always have separate shipping and haz mat fees, and that makes sense. I've gotten a bunch of 'free shipping' but never free haz mat costs.
I believe the outdoorlimited billing program just didn’t cover the break down of shipping/haz mat fees properly when I put the free shipping code in.
Anyway, I just got off the phone with them too. I didn’t have any problem paying the haz mat fee. In effect, the shipping portion is still free, the site just didn’t put the haz mat fee into the original order, and so we fixed that oops. $20 on my order. Everything seems above board to me.