SELECT DISTINCT(aps.customer_id), 
       rc.customer_number,
       rc.customer_name,
       SUM(aps.amount_due_remaining)
FROM ar_payment_schedules aps, ra_customers rc 
WHERE aps.customer_id = rc.customer_id
  AND aps.class IN ('CM', 'PMT', 'INV')
  AND aps.status = 'OP'
  AND EXISTS (
         SELECT a.customer_id
         FROM ar_payment_schedules a
         WHERE a.customer_id = aps.customer_id
           AND aps.status = 'OP'
           AND aps.class IN ('CM', 'PMT', 'INV')
           AND TO_CHAR(aps.trx_date, 'J') <= '30-SEP-95' 
         GROUP BY aps.customer_id)
GROUP BY aps.customer_id, rc.customer_number, rc.customer_name;