You can ship the barrel for any gun through the USPS or UPS it don't matter. It is the frame that you are suppose to ship through a FLL legally. That way the government can keep track of who owns the gun? But the T/C custom shop told me I could ship my contender receiver to them through the mail and it was OK. So I would say you could probably just ship them seperate and it would be OK.
But if you wanted to do it 100% positively legal you would have to ship the frame to a FLL and have it registered with the new buyer!! The barrel could be shipped like a normal package.
Hope this helps. I have bought numerous barrels through the mail and have not had any trouble. If it was me I would just mail them seperate.