On package label, it's best *not* putting any word associated with "gun". Some FFLs were issued to the businesses with names like "ABC Gunsmith" or "XYZ Pawn Store of Jewelry and Guns"...... no, no
By package of this size, this weight, destinated to this address, what can be in the box...... those things have resale value so very easy to get lost in transportation. Plain label, package well, and shipping via Fedex or UPS should work.
====
That's not required by regulations. Just for seller and buyer's own goodness.