Ethan mentioned being contacted by the FBI about packages that he was going to received. I am not sure if it was in relation to the skulls, but I think the two being connected makes a lot of sense.
I also think that the seller may be very experienced about proving that it is for research (i could be completely wrong here, as I have no clue how this works)
Also, upon researching more, it seems the website selling human remains is 100% legit (although with dubious ethics), and has definitely been in business for a long time
Here is a Forbes article from 2015 about it selling human remains:
L /
A
This is a Reddit
post from 2 years ago about a human rib bought for a friend, with USPS First Class shipping
There are also many other posts around the internet, such as hunting forums, that talk about Skulls Unlimited
View attachment 7257362
(Edit: previous post on this with more info on the company:
https://kiwifarms.net/threads/anisa...e-washburn-anisas-husband.38107/post-21184657)