I'd hold the seller/shipper's feet to the fire.
They should've removed the strings completely, then used some corrogated cardboard forms and duct tape to secure the headstock.
Nonetheless, it is repairable. You want to glue that without adding extra junk, like splines, pins, screws, etc.
After removing all the hardware, you could devise a workboard with a precisely angled wedge to support the headstock.
Or, another approach is to use a heavy "non-stretch" tape in place of a workboard jig. Binding tape, as used in guitar construction, resists tearing and stretching.
The idea is to join the two parts in exact position, then lay a strip of tape about 6-8" long that runs from the first few frets of the fingerboard, over the area for the nut, and a few more inches over the face of the peghead. Make sure it is closely pressed to all surfaces and right into any corners. You should now have a "hinge" that can open up just wide enough to squeeze in some glue, then close back up with perfect fitting. The tape also prevents the parts from slipping once clamped.
Do dry runs w/o glue to be sure that the two parts align perfectly. The key to a successful glue joint with this type of damage is precise fitting of the parts.
High pressure clamping is not needed, just some firm pressure for glue squeeze out and to hold while it sets.
I would use a single wooden cam clamp, and just enough pressure to get glue squeeze-out.
Both the binding tape and cam clamp are available from Stewart-MacDonald.
I would use hot hide glue, but most guys would likely choose Titebond.
Hot hide glue gives a very limited assembly time (like, 30-60 seconds). But, when parts fit perfectly, you will get a nearly invisible seam.
Glu-Boost makes nice products for finish repair work.
I have done many, many such repairs. It's not difficult, but does require planning and patience. Good luck with this one!
