So I declared I had a rifle this time. I called ahead and they asked me to declare it so I figured why not.
There was a clerk there who had some firearms familiarity, he took it into the back and checked the chamber. It was a big sloppy 50 cal so I could hear the bolt open and close. They never opened it at the counter, which is probably a good way of doing things.
He said he had never shipped a firearm before. I said, "you've never shipped a firearm that
you knew about before".

Attached is the language used for the shipping disclaimer and how they shipped it. In practice, declaring it saved me quite a bit of money because I wasn't obliged to put it in a cardboard box, which would've added a few inches and pounds.
They shipped it Priority Mail, I paid the standard Priority Mail rate with signature confirmation. They never asked me if I wanted to put additional insurance on it, which was probably an oversight. The receipt doesn't have any special "this was a gun" line item, nor did they put any special labelling on the case. Just another package as soon as it was in the system.
It worked out. FFL sent it back to my doorstep a couple weeks later.
Unless somebody presents some significant evidence, I'm pretty convinced that there is no compelling reason or obligation to declare you're shipping a long gun.
Afterwards I asked the clerks there if they wanted me to declare, if I had to declare and if that was the proper process... They said yes, because they had to check the chamber. But they also told me three times that I had to put "IN CARE OF" on the package, which I repeatedly corrected "no, I think that's only if shipping intrastate", and which they then agreed upon... So they don't seem to have any training or knowledge beyond the same on-line website language that we are operating off of.