можно и короче

SELECT i.billing_country AS country,
       COUNT(i.invoice_id) AS total_invoice,
       tc.total_customer
FROM invoice AS i
LEFT JOIN (SELECT billing_country,
        COUNT(DISTINCT customer_id) AS total_customer
        FROM invoice
        GROUP BY billing_country) AS tc ON i.billing_country = tc.billing_country
WHERE EXTRACT(YEAR FROM CAST(i.invoice_date AS timestamp)) = (
        SELECT EXTRACT(YEAR FROM CAST(invoice_date AS timestamp)) AS invoice_year
        FROM invoice
        WHERE EXTRACT(MONTH FROM CAST(invoice_date AS timestamp)) IN (6, 7, 8)
        GROUP BY invoice_year
        ORDER BY SUM(total) DESC
        LIMIT 1)
GROUP BY country, total_customer
ORDER BY total_invoice DESC, country;