I bought a guitar from a guy on ebay that got broken in transit - check out this thread. Basically, you do need to worry about it. You can fix it yourself if the wood is all still there - just wood glue and clamps and it's very easy, but you have to do something - with the holes in the neck for the locking nut, there isn't much wood there so it could well get worse under string tension and split more. On mine, the rosewood 'board had started coming away from the neck as well.
Whether to repair it or get a refund is a call you'll have to make - I don't know how good a deal that is for a JS100. If you decide to do it yourself, it should be possible with:
wood glue
some fine sandpaper / glasspaper
clamps
danish oil or similar for refinishing
a couple of rags for tidying up and rubbing down
But whether or not you take it on depends on how confident you are with that sort of thing. Good luck, and let us know how it turns out!
**EDIT** The cracks will presumably be what remains of a very significant injury! Probably your guitar looked like mine in the link above but far worse - the cracks in mine were never more than an inch or so!
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