Well, the entire procedure requires you to first trust the snail mail chain in the first place, so it's a different category of trust that "trust a CC provider". Snail mail used to be sacred, but it's been known not-to for a long while now. And at the point that you can expect the acabs are willing to inkdust and laser your mail for biological traces, that means you are facing a nation-state adversary with nation-state power, so you should be looking into nation-state level defenses instead.