As far as using these bits, I have only been using the React lib so your mileage may vary if you decide to go Vanilla JS. To get started we need a host for the Argo Checkout extension and that's where the limitations of not being part of closed beta begin. Besides picking up remote-ui and writing your own remote environment mocking parts of Checkout::PostPurchase::* (which you can by reading through here and looking at this example) we will have to make dos with what is publicly available. The only remote environment I am aware of that devs can use right now and Shopify devs (thanks Chris Sauve) confirmed we can use is