It’s a bit of a grey area, as herbal medicine is also unregulated in the US. However, the NPN’s only are a requirement in Canada, so don’t worry about applying for them just because you want to ship to the US. My suggestion would be to keep your selling within Canada, and researching requirements within the US if you want to sell more to that audience down the road.
In my experience, I have sent some products to the US but have not marketed it on my website (thus, it’s only been a handful of times). On my website, I only offered shipping within Canada. When I did ship to the US, I made sure to keep the box very minimal (eg. I did not include my Sisterhood Apothecary tape, as I didn’t want the box to get flagged), and I just wrote herbal tonic or herbal elixir on the customs form (which you can do before going to the post office online). I haven’t had any issues personally. You can opt to get the package shipped back to you in case there is a problem, when you fill out the customs form.