update checkout handler

cmd/cart/pool-server.go

@@ -488,6 +488,24 @@ func (s *PoolServer) SubscriptionDetailsHandler(w http.ResponseWriter, r *http.R
 	return nil
 }
 
+func (s *PoolServer) CheckoutHandler(fn func(order *CheckoutOrder, w http.ResponseWriter) error) func(w http.ResponseWriter, r *http.Request) {
+	return CookieCartIdHandler(s.ProxyHandler(func(w http.ResponseWriter, r *http.Request, cartId cart.CartId) error {
+		orderId := r.URL.Query().Get("order_id")
+		if orderId == "" {
+			order, err := s.CreateOrUpdateCheckout(r.Host, cartId)
+			if err != nil {
+				return err
+			}
+			return fn(order, w)
+		}
+		order, err := s.klarnaClient.GetOrder(orderId)
+		if err != nil {
+			return err
+		}
+		return fn(order, w)
+	}))
+}
+
 func (s *PoolServer) RemoveVoucherHandler(w http.ResponseWriter, r *http.Request, cartId cart.CartId) error {
 
 	idStr := r.PathValue("voucherId")