I searched everywhere for this using all the keywords I could find, and I cannot find anywhere where people discuss how to make an NPC walk through a load door...

I feel like this has to have been discussed somewhere because it seems pretty standard, but I was too embarrassed to ask, so instead I just made a package to have an NPC walk to a load door and then used triggers and markers to teleport from one side to the other.

I don't like the way that looks and its glitchy, so I am breaking down and asking the stupid question... how do I do this?

First off make sure your navmesh is finalized (should be a green triable very close to said load door)
second a travel pack, so the said npc travels from one cell to another (eksample would be to carry wood from one pile to another pile in anothe cell, or travel from first cell to an inn)

basic rundown of the verious AI packages in this video http://www.youtube.com/watch?v=E3Nd8LXYRA0