Yes, you can 100% ship a long gun to an FFL via the USPS. Many, many postal worker's don't know their own policies. I've had to print them out and take the to various post offices in the past. I even had an insurance claim denied once because as the letter stated "it was illegal to ship a firearm through the USPS". I contacted a higher up in the post office and received my check shortly after. Many postal workers have told me that they just aren't trained properly.
