Canadian Selling to the US, Need EIN?
If your company produces profit in the US it would be subject to US taxation. That means it would need an EIN, and since you are shipping form a location in the US you create nexus in the state where the warehouse is located. That also means your company would be obliged to collect sales tax on tangible merchandize sold to residents of this particular state (assuming this state has sales tax).
To better understand how foreign companies doing business in the US are taxed I invite you to attend our free tax webinar for non-US entrepreneurs.