Once living in BirchBay, just below Blaine, my son and I had gone to the International Market for some munchies. We were watching Saturday cartoons, and toking. Like an idiot, we decided to take a different route back to the house, and inadvertantly found ourselves on the Canadian side. Realizing our mistake we tried to make a U-turn back into the states. That's where we ran into problems. The problem came because my son didn't have any ID on him, I was driving, and it was just to be a short trip to the store anyways. They brought us inside and searched the car thoroughly, finding nothing, of course they didn't, it was in my pocket!!, but they did detain us for a long long time. My son has a history, and they were just getting all the info from the states we'd lived in. It was a bit unnerving to sit there and watch them eyeing the computer, and occasionally looking up and at us with a look of OOOOHHHH UH HUHHHHHH. This was before 9/11 and I thought the US side was pretty scary then. Seemed like we were drawn into some sort of international intrigue and we were just on a mission for munchies. Be sure to have all proper paperwork, pleasant demeanor, and don't be nervous.
You might try the crossing to the east of Peace Arch, it might be the truck crossing, and I think it's just above a place called Lynden, but it's been years since we lived there. Can't remember