Actually, it seems that your concerns are with US Immigration - customs is what you declare that you are bringing into the country and the customs duties (taxes) you have to pay to import something.Immigration is showing your passport and explaining why you are coming to the US. Yes, the customs officer will ask where you are headed and how long you will be there, and your best get is to always tell them the truth. So yes, you should tell them that you are headed to the Tulalip outlet malls to do some shopping and are planning on driving back to Canada that same day.
yes a lot of people from come to america to go shopping!! but you can only buy so much a day here without having to pay tax on it when you go back? check at the border and they will tell you how much you can buy a day??