My son got caught with 1 joint in Blaine Washington...
We lived in Canada and the Canadians would not let him cross to come home until all his court stuff was over so we had to pay a guy to sneak him in on a boat. We lived on the water. After court when he was found guilty they said it was to their discretion as to if he could cross. Sometimes they let him other times they didn't. We complained to the main office but it did no good.
My son has a smart mouth though...so just be very polite and no matter how mad you might feel don't let them know it. There are some nice guards too.
Make sure you keep your rental car receipt...

Whatever you do don't have any weed or pipes or anything on you. No alcohol and absolutely NO WEAPON of any kind.
They will ask you if you are bringing anything into Canada...if you have luggage then say you do...they will probably want to go through it.
It will be the same thing going back into the US on the other side.

Canadian cops are no where near as anal as the border guards...You will have fun once you are across
but the Blaine police on the US side are terrible. They will give you a ticket for anything including driving too SLOW...
let me know how it goes...so where are you going in BC?