It has been a while but when I last shipped a shotgun, i printed a copy of the USPS regs, included a copy of FFL information inside the box and under shipping label.
They don't often see them shipped from private citizens and their first inclination is to look for a way to say no. I bought the insurance as well. I once had a gun shipped to my FFL holder that took 12 days to arrive......I had filed a claim before it showed up - anti-gun folks wont risk their job but will make it tougher if they can.
