Start a Stripe Checkout session to save a payment method

Opens a Stripe Checkout session in setup mode: it saves a card WITHOUT charging it. This is the card-on-file flow that agent purchases and domain buys draw against later; it is not a plan purchase (see /billing/checkout for that). Requires billing:manage (owners and admins). Supply EITHER return_url for the embedded flow, OR both success_url and cancel_url for the hosted redirect flow. Supplying neither is a 400.

ui_mode is optional and picks how the session is shown. Leave it out (a null or an empty string counts as leaving it out), or send embedded, for the behavior from before it existed: Stripe Embedded Checkout when return_url is set, and the hosted redirect when only success_url and cancel_url are. elements is Elements with the Checkout Sessions API: the caller renders the whole page and mounts only Stripe's Payment Element with the returned client_secret, so the page can wear its own theme. It needs return_url, which is where redirect based payment methods and the finished setup land, and the session is created on a Stripe API version of the dahlia release train (the billing guide names the exact version), so mount it with a Stripe.js of that train. The response's ui_mode says what was really created (embedded, elements or hosted): an elements request answers embedded when Stripe could not create an Elements session, so read it before mounting anything. hosted is only ever an answer, never a request value: a client that keeps the response's ui_mode must not send it back (the redirect flow is chosen by sending success_url and cancel_url without return_url). Any other value is a 400.