the user's existing customer (reused so a saved card isn't orphaned), or null to let Checkout create one.
the URL Stripe returns to on success (the app composes it from its origin + route).
the line-item product name shown on the hosted page, e.g. "acme — 600 credits".
the user's existing customer (reused so a saved card isn't orphaned), or null to let Checkout create one.