Choose Nexus code C31 and then change to C32 after establishing an office. C31 is very broad: "A foreign entity or organization that has a bona fide presence in the United States of America or any of its possessions or territories who regularly engages in lawful activities, sales of goods or services or other business, commercial or non-commercial, including not-for-profit relations in the United States." so basically if you sometimes sell something to US citizens, you qualify. And besides I don't think it's tightly enforced.