There's an old saying..."it's never a problem until it's a problem". Shipping a barreled action as "machine or rifle parts" might seem like a good idea if everything goes smoothly but what happens if it gets lost and you have to file an insurance claim for a "firearm" that you did not declare? Since this declaration is something carriers require, you could have a problem. Personally, I would not be less than truthful about what you are shipping.
If your gunsmith is a FFL, he can accept shipments of your "firearms" from you and send them back to your home address. You do not need a FFL for a firearm you are having returned.
As far as shipping methods, long guns are NOT the same as handguns and can be sent via USPS, UPS or FedEx. They can be shipped ground and not have to be shipped overnight. UPS and FedEx do require that you take your boxed firearm to a "hub" for shipping. This means no local mailing store or Kinkos.
As for packaging, I've used boxes and gun cases inside cardboard boxes with success. I never have any wording on the outside of the box indicating the contents. I also do not use any gun-related terms in the address label such as gunsmith, armory, tactical, etc., merely the name of the person or initials of the company.
Hope this helps...