Requires `checkout:write` scope. **Requires `set_shipping_address` to have been called first.** Returns 400 `invalid_shipping` if no address is on the cart, or 400 `delivery_unavailable` if the buyer's address is outside the seller's coverage. Cart total must not exceed the platform cap (`kifly://platform/limits`). **Checkout is always human-in-the-loop: it returns a link the buyer opens and confirms — it never charges a card or completes an order on its own.** On success: `payment_rail` tells you which outcome you got. `'stripe-checkout'` — a real Kifly purchase: returns `payment_url` (the Stripe link the BUYER must open and pay), `session_id` for `order_status`, and amount breakdown. **Relay the link and tell the buyer they need to complete payment themselves — do not imply the purchase is done until `order_status` reports `paid`.** `'directory-handoff'` — this seller is discovery-only (`kifly_purchasable: false` on the cart, as already signaled by create_cart/add_to_cart/get_cart); there is NO `payment_url`/`session_id`. Instead a `handoff` object carries `seller` (name, handle, storefront_url) and `items` (each with a tracked `link` to the product on the seller's own site, or null — fall back to `seller.storefront_url`), plus a `disclaimer` to relay verbatim. Present this as a referral to complete the purchase off-platform, not as a completed Kifly order. **To pre-fill the buyer's email on the payment page, pass `email` — ask the buyer for it in plain language ('what email should the receipt go to?'). Never ask the buyer to paste a token.** `buyer_token_status` in the response reports whether an (internal) token was applied to pre-fill the saved email: `'resolved'` / `'invalid'` / `'none'` — an invalid token is NOT an error response. On 502/503 errors, check `retryable: true` in the body — those are transient upstream faults; wait `retry_after_seconds` then retry once.
Connector