What payment method did that customer use?
SELECT t1.payment_method FROM customers AS t1 JOIN customer_orders AS t2 ON t1.customer_id   =   t2.customer_id JOIN order_items AS t3 ON t2.order_id   =   t3.order_id GROUP BY t1.customer_name ORDER BY sum ( t3.order_quantity )  LIMIT 1
payment method used by a customer refers to t1.payment_method; identifying a specific customer involves joining customers, customer_orders, and order_items tables based on customer_id and order_id; the customer in question is determined by the highest sum of order_quantity;