Tell me the payment method used by the customer who ordered the least amount of goods in total.
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