Create a Stripe Checkout Session with Sphere tax applied

Send Stripe's Checkout parameters under stripe, and the buyer Sphere needs to price under sphere. Sphere calculates tax, expresses it as Tax Rate objects in your account, creates the Checkout Session with those rates and returns it. Redirect to url (hosted) or mount client_secret (embedded) exactly as you do today. When the session completes, call the complete endpoint (or let Sphere's webhook do it) so renewals are taxed by Sphere.