select cust_name, t1.order_num, oi.OrderTotal
from Customers join Orders t1 using(cust_id)
join (select order_num, sum(quantity * item_price) as OrderTotal
        from OrderItems 
     group by order_num) as oi
     on t1.order_num = oi.order_num
    order by cust_name, oi.OrderTotal