A letter of apology to customers regarding incorrect product delivered. It offers delivery of replacement goods or reimbursement to customer, and provides explanation of the error made.
We are extremely sorry to find out that the goods delivered were not the ones you have ordered on [date]. [The replacements are being sent [freight prepaid] to you right now and should arrive at the delivery location by [date].] [Please return the incorrect goods [to the same shipper] / [by freight and we will reimburse you for the charges].]
It is purely a [handling] / [system] / [process] / [sales] error. [We are reviewing our process to see how we can avoid this type of error in the future. We hope that these steps will ensure the correct delivery of the goods to our customers in the future.]