I don't know how the US does it, but at least where I'm from, major border crossings (be it a road or an airport or something else) typically always have a place you can go to declare goods you're importing. That's where you're typically expected to, well, declare your goods. I imagine they don't see many people just driving there and saying "Hi, I'm here to declare this car I'm in", but I don't see why it shouldn't work...