Went through Port Huron recently and exported a vehicle. It's a bit tricky but I'll tell you what I learned.
But first of all, the CBP people I met were excellent. I was pretty much lost trying to find the office so I asked an officer who was walking across the parking lot where to go. Rather than just tell me (it was complicated) he walked me over to the right place after first making sure I had the correct paperwork (i.e. the title and an ITN). The guy at the counter inside was pleasant and efficient. No arrogance or rudeness to be found. YMMV.
At Port Huron, you need to understand that the CBP office is setup for trucks incoming from Canada. It's not actually part of the outgoing traffic pattern headed to Canada so here's what you need to do.
As you approach the toll booth for the bridge to Canada, immediately get way over to the left and parallel park beside the concrete "jersey barriers". Do not go through the toll booth! Get out of your car, climb over the barrier and look at the truck loading dock on the back of the building in front of you (the CBP building for incoming trucks from Canada). On the right end of that big long loading dock is a steel door with a sign on it that says "Driver's Entrance". Go up the stairs to that door, enter, turn right, and walk 50 ft down the hall to the door at the end of the hall that says "Wait here". There may be a line up but when your turn comes, look through the glass in the door until you see a CBP officer waive you inside. Give him your title and ITN and you will be out of there in five more minutes.
FWIW, I understand the Detroit CBP office is much harder to find.