I haven't shipped a box turtle, but I have shipped a painted turtle.
I put her into a rubbermaid container, with holes poked thru it. I packed damp shagnum moss around her, to keep her moist, and prevent her from being jostled around too much. I would think that dry sphagnum moss for a box turtle would do the same. Make sure that there is not a lot of extra room in there for her to slide around.
I didn't mark on the box what was in it, but I marked FRAGILE< THIS SIDE UP< HANDLE WITH CARE and whatnot all over it, as well as what temps it should be kept between. Overnight delivery is a must.
Good luck with that!
-----
GratefulFred