I've long ago taken to shipping long-arms broken down, with action and stock separately wrapped and padded. This gets it into a box that doesn't scream 'RIFLE' due to it's length. Plus, it saves you money on an oversize package surcharge at USPS. Less danger of damage also, from the impacts leveraging wood against metal when they're slinging things around.
Personally, the only time I was stalled was when a new clerk had to check with a supervisor. He just asked if it was a rifle, and not a handgun.
When I affirmed, he told he it was perfectly fine to continue.