SAMPLE - EXPLANATION FOR DELAY OF PARTIAL SHIPMENT

After some investigation, I believe that we have found the source of the problem that led us to our misunderstanding on     [date]

When we received your purchase order for [quantity, product], we were unable to fill the order for immediate delivery due to     [explanation for delay in delivery] Our letter informing you of this delay and requesting your
instructions was mailed to you on    [date]    We did not receive a reply and proceeded to send a follow-up letter to you on     [date]

We have been able to make delivery to you since the [date] but felt that since you had emphasized "for immediate delivery" on your purchase order, we should wait for your authorization before shipping.

I can easily understand why you were so upset with us [day] if no one made you aware of our correspondence.  We have always appreciated having you as a customer and hope that we can continue to provide you with our products in the future.