Start Checkout

Create a hosted checkout and return its URL for the client to navigate to.

Returns the URL rather than issuing a 302. A redirect from an XHR is followed by the browser and lands the JSON response in a fetch the client cannot use, so the client has to navigate. Being explicit about that here beats a redirect that happens to work only because someone used a form post.

Already-Pro is refused with 409 rather than quietly making a second subscription. Two live subscriptions for one account is a double charge, and the honest answer to "buy Pro" when you already have it is "you already have it".