Thanks for the feedback Ryan. I may not have been very clear in my response - I know I was bouncing all over the place. usa cannot be my base URL for scalability reasons. Slowly we'll start adopting other countries websites so the top-level subdirectory needs to be broad enough to not restrict us to a specific part of the world.
My intitial thought is to opt for something like website.com/M/us-en or website.com/-/us-en where the base directory is short in hopes that it doesn't dilute the value of SEO terms later in the URL such as website.com/M/us-en/store/product-name.html. Is that something to worry about?