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.