As of today. It is not possible to publish the "single app" in the store, but we have already some users selling subscriptions through their website. After the checkout and payment on your website, you would "unlock" the app for this customer by inviting him to the app. Only then would you have to buy a license for this private user.
We provide the possibility to get a branded version of the Open as App container app. This is a payed feature on top of the enterprise subscription (see more details here: Pricing & Packages). This app can be submitted in the store.
One thing that might be interesting for you - if there is only one app installed/created in your account, this app will open after clicking the Open as App icon on the home screen of your device.