instruction: What is the payment method of the customer that has purchased the least quantity of items?
response: 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