Need some help with shipping issue, gun gone missing from package.

So basically quick summary:

I have this ksg I finally sold that I put in a box and I take it apart to fit in the box (buyer is cool with it), box arrives all fucked up and whole barrel assembly and serialized portion is missing.

I do not have serial number written down (idiot)

I am a private seller who shipped it to the FFL with a copy of my DL.

I did not declare it a gun when I shipped it from UPS.

What should I do and what are my next steps? How fucked am I here?