The import process is a bureaucratic nightmare - you don’t send it through the post in any case, lest it get lost for long enough that your samples perish.
The more common route is you complete the paperwork, fly with it in your carry-on and declare it all at customs.
Having worked in a related field, it certainly wasn’t uncommon for scientists to skip some of the hassle and just not declare their research material. Definitely not ‘proper’ and certainly higher risk to their immigration/visa status if they were foreign nationals, but still not uncommon when the alternative is spending hours in CBP inspections before you spend hours in the normal immigration queue.
The more common route is you complete the paperwork, fly with it in your carry-on and declare it all at customs.
Having worked in a related field, it certainly wasn’t uncommon for scientists to skip some of the hassle and just not declare their research material. Definitely not ‘proper’ and certainly higher risk to their immigration/visa status if they were foreign nationals, but still not uncommon when the alternative is spending hours in CBP inspections before you spend hours in the normal immigration queue.