Dear Sirs,

I am writing to complain about the shipment of wooden furniture which we received yesterday. It was two weeks late. This is the second time we have had to write to you on this subject and we cannot allow the situation to continue. We have already explained that it is essential for the order to arrive on due date as late delivery could create very serious problem.
Unless we get assurance that you can guarantee promptness of all future deliveries, we will have to look for another supplier. We need your confirmation before we place our next orders.

Yours faithfully,

John
Manager