Perhaps this was your issue......
The box of the iPhone is hard. If you placed your phone face down or even face up in the box (even when said box is bubble wrapped), and this is placed inside another box, if the whole contents is overly solid, any heavy weight placed on top (i.e. during transit) is gonna be fed through to the contents inside.
Sometimes people can overwrap stuff which causes so many layers that it actually creates less safety inside.
I'm afraid to say that if the IMEI matches then it 100% IS the same phone you sent them (which, has unfortunately, become damaged in transit).
I can't see any way out of this and I would be miffed if this was me also.