There is an exception for occasional importation which means if you're in the business of importing and are licensed to do so you must stamp, but A local gun shop that doens't do imports can still import a gun on occasion for a customer and are not required to stamp because they're not a "licensed importer". Of course using a shop that's never done it before can also cause hiccups if they don't know the process. I'd be more comfortable with a place that does it all the time. Pick your poison I guess.
From the ATF site:
Q: May a licensed dealer who does not have an importer’s license make an occasional importation?
Yes. A licensed dealer may make an occasional importation of a firearm for a non-licensee or for the licensee’s personal use (not for resale). The licensee must first submit an ATF Form 6, Part I to ATF for approval. The licensee may then present the approved Form 6 and completed ATF Form 6A to U.S. Customs and Border Protection. Contact the Bureau of ATF, Firearms and Explosives Imports Branch for forms, or download them from the ATF Web page at
http://www.atf.gov/forms/firearms/.Marking for LICENSED IMPORTERS:
Link