APOLOGY FOR INCORRECT SHIPMENT
I was deeply embarrassed by your [STATE FORM OF COMMUNICATION, ex. telephone call yesterday]. Before apologizing, however, I want to assure you that I will not allow such an error to recur; our company has a favorable record of customer satisfaction and I will not allow that to change.
[STATE WHAT HAS OR WILL BE DONE, ex. Immediately following our conversation, I ordered our delivery trucks to arrive at your premises tomorrow to deliver the missing products and pick up the incorrectly-shipped items.] I apologize for the inconveniences this must have caused you. Please call me at XXX-XX-XX if there is anything I can do for you.