How about their e-mail addresses?
SELECT T1.email FROM customers AS T1 JOIN invoices AS T2 ON T2.customer_id   =   T1.id ORDER BY T2.invoice_date DESC LIMIT 5
e-mail addresses refers to T1.email; customers JOIN invoices to connect customer data with their invoices; ORDER BY T2.invoice_date DESC to sort by the most recent invoices; LIMIT 5 to limit the result to 5 entries;