I find it easier just too avoid the hassle and questions and label it as machined parts.
Blessings
That is a good idea. But seriously, your mail is your mail, and is federally protected. As long it is not on a list of items that much be labeled (firearms, ammo, explosives, chemicals, etc) then they have no right to ask you what it is. They can ask you if it is a chemical, explosive, or firearm, and all you have to do is say no.
If they push the issue, THEY ARE BREAKING FEDERAL LAW. Call the FBI, have them busted.
As long as it is not a restricted item, you can mail what you want when you want to, without hassle.