Dear

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.