Buying goods in Canada and bringing them across the border to US?

Long story short I can buy ductless AC units in Canada for $800 that cost $2700 in US with shipping. So I’m considering driving to Canada in my van and buying 3 units for personal use, not resale. Will I have problem at the border? Do they check what’s inside van? Is there some paperwork I need to fill out. Please be specific. Thanks