There has been an update to the importation of vehicles into the US. They are no longer free of duty. I am guessing parts are the same way. If you are taking cash to pay for them, make sure you tell Canada Customs how much you have if it's close to $10k after converting to Canadian funds. As far as being open on weekends or 24/7, that depends on the port you will be crossing. I would recommend you calling the port and see how they handle the process and hours.