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' 。
Referring to your inquiry letter dated 29th September, we are offering you the following subject to our final confirmation.
关于贵方9月29日的询价信,我方就如下产品报价,以我方最后确认为准。