From Section "E" of the linked info:
"any package or other container in which there is any firearm or ammunition without written notice to the carrier".
Here's the relevant Federal Law on shipping firearms by any carrier:
ATF SHIPPING LAW
A nonlicensee may ship a firearm by a common or contract carrier to a resident of his or her own State or to a licensee in any State. A common or contract carrier must be used to ship a handgun. In addition, Federal law requires that the carrier be notified that the shipment contains a firearm and prohibits common or contract carriers from requiring or causing any label to be placed on any package indicating that it contains a firearm.
[18 U.S.C. 922(a)(2)(A), 922(a) (3), 922(a)(5) and 922(e), 27 CFR 478.31 and 478.30]
Note the "carrier be notified that the shipment contains a firearm".
Failure to notify the carrier the package contains a firearm or a serial numbered frame is a Federal offense.
Second reason not to lie:
If the gun is lost, stolen, or damaged the carrier will not pay you a dime, because you lied on the shipping form.
The shipping form is a legal contract. If you tell the shipper the package contains "machine parts" you've broken a legal contract and defrauded the carrier. They own you nothing and will give you nothing.
Think you can lie again when they ask what KIND of machine tools?
Don't you think they might get just a little suspicious when the address the package was shipped to is the address of a gun maker?