If you don't know a person's last name, use 'ma'am' or 'sir'.
如果你不知道一人的姓, 你应称呼他为 'ma'am' 或 'sir' 。
Provided you can offer favorable quotations and guarantee delivery within four weeks from receipt of order, we will place regular orders with you.
贵方若能报优惠价并保证在收到定单后4周内交货,我方将定期订购。