Glad to hear that you've got your plan figured out. There is a lot of misinformation to navigate. I moved to Hawaii earlier this year with 2 rifles, and mailed them to myself (care of my father, who already lived here) via registered mail. I had the boxes each rifle came in, and I put both of those in a golf club box from the UPS store, and filled the empty space with packing peanuts.

Here is what I know about sending a firearm via registered mail:
- long guns only, handguns are not allowed
- guns must be unloaded, obviously
- guns must be shipped separately from ammunition
- box must be sealed with paper packing tape, no cellophane tape.

I took my ammo on the plane in my checked luggage. You have to make sure you declare the ammo to the check-in desk. TSA weight limit is 11 lbs of packaged ammunition, no loose rounds allowed. I did my best before packing to weigh out 11 lbs with the fishing scale I had, and one of my friends scored a box of CCI standard.
The TSA guy weighed it out to 11.2 lbs while asking me the standard profiling questions. He was a bro and let it slide. I had read that Hawaiian Airlines had a ammo weight limit of less than 11 lbs, but it didn't affect my experience.

To register your out-of-state purchased long guns with HPD, you need to go down to the Beretania St. station. My understanding is that you have 72 hours to register once you *take possession* of the firearms.

Oh, and welcome!