I know you don't understand but you will just have to accept my opinion this time.
我知道,你或许不能理解,但这次你必须接受我的意见。
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。