I ship air guns via USPS all the time. Federal law mandates that an air gun IS NOT and CAN NOT be classified as a firearm, therefore, no restrictions on mailing as long as you don't try to mail a filled gas cylinder of any kind. Problem is, most postal employees don't know their own rules, it's right up there with the IRS. Bottom line, as long as I'm shipping something I know isn't hazardous, what's in the box is none of their damn business. This is a classic case of what the moron behind the counter doesn't know won't cost you money :-)! Later.
Dave