I bet you a jam sandwich that an NSL is not actually a letter sent in the regular post.
Yahoo was able to publish the letters they received: https://s.yimg.com/ge/tyc/Redacted_Non-disclosure_Terminatio...
But in every company I've worked for mail is signed for by whoever and, if it's addressed to an executive, delivered to a secretary who reads it and decides what to do it.
Given the requirement for secrecy, there is zero chance that an NSL will be treated in this way. And it won't be served by a random process server. Most likely it will be served by an NSA employee, in a discreet situation of their choosing.