Question: What is the payment method of the customer that has purchased the least quantity of items?
SQL: 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