Can`t recommend a particular carrier/courier, cos if were down to me, I`d be scared of what any of them would do.
I`d just do your damndest to protect it in wrapping, and secure some things on the inside of the case perhaps. Don`t want the gfxcard knocked out or anything.
(don`t get a can of expanding foam and squirt it in to
doesn`t work - looks good on utube tho)