I thought you needed a FFL to be the entity to send out any firearms out from Hawaii. But I could be mistaken.
But I do believe that US Postal REGISTERED is the most secure. I researched this and spoke to US Postal, who advised that only certain pre-approved US Postal Employees are authorized to have access to their REGISTERED mail parcels. Plus, US Postal keeps track of which US Postal Registered-Approved employees had actual access to the registered parcels. They sign in and out of the REGISTERED mail areas. An associate of mine worked as a US Postal Registered-Approved employee, and told me the same thing.
With US Postal Registered you also have to do that brown paper wrapping with that water-activated brown tape (not the regular already-sticky standard brown packing tape). I preferred the fiber-reinforced water-activated brown tape. You are basically wrapping your parcel like a Christmas present, and then completely sealing the entire length of every single seam with this water-activated brown tape. And of course you got to afix the greed Return Receipt Signature card that the recipient signs. And then the receiving US postal clerk stamps each seam. And I do not thing US Postal's Registered mail can be sent their faster method of Express. When we price compared to FedEx, US Postal REGESTERED was more expensive.
If you go through an FFL, you should request that the FFL use US Postal REGISTERED (which I think automatically includes their signed return receipt). Note that US Postal Certified (which may only include tracking) is not as secure as US Postal REGISTERED.
I do not think FedEx has any kind of special secured option similar to US Postal REGISTERED. Although each FedEx parcel can be tracked, it seems comparable to US Postal Certified.
But hey don't take my word for it. Please call US Postal. When I did, they were very helpful. And if I am mistaken, please post and let me know.