There is something that is misunderstood. There are different definitions that apply to what is a firearm. To the BATF, the frame is the firearm and would be classified as rifle, handgun, etc. To the U. S. Postal Service, a frame is a "gun part" irrespective of what its use was or is going to be. If it doesn't have grips or short barrel attached and doesn't look like a handgun, the U. S. Postal Service won't have a problem with you shipping it.