Display the 10 least expensive invoices | Would you like a list of the id's of these invoices? | No.
SELECT T2.invoice_date FROM customers AS T1 JOIN invoices AS T2 ON T2.customer_id   =   T1.id ORDER BY total LIMIT 10
least expensive invoices refers to ORDER BY total ASC; display the 10 corresponds to LIMIT 10; invoice date is requested;