I have received and shipped antique style muzzle loading rifles with no problem. I use click and ship from home. One was even in a plastic rifle case. The questions asked do not include firearms. Any firearm is possible to be shipped without so identifying. I'm not saying handguns would be legal but if shipped without identifying it is not likely there would be a problem.
If taken to a PO that is another story. Bureaucrat clerks like to exercise power and you could be required to identify contents even if the law doesn't. And you could be denied even if the law allows. Arguing will only cause you frustration.