That was another solution I considered. I think that would work, but it seemed expensive to implement. I believe it would still require me to host a separate copy of my app or a standalone app that handles just payments. Stripe also creates its own child iframe for displaying payment UI, so at that point, my app would look like:

  Main app
    My sandboxed iframe
      Stripe's iframe

So managing all the bubbling up and down of messages felt like it was going to be complicated. Limiting Stripe to a single page and forcing the new HTTP requests to unload it is a bit hackier, but is really simple to implement. You can see it in the blog post - I only had to add ~5 lines of extra code to my app to make it work.