Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [customer name],
Thank you for your [correspondence/phone call] of [date complaint was made]. We are very sorry that you received the incorrect order, and have rushed a replacement order to you. In your replacement order, we have also included a postage-paid label. You need only attach that label to the box you received with incorrect products and give it to your mailman.
We value your business, and apologize for any inconvenience this may have caused you. Please be assured this will not happen in the future.
If there is anything else we can do for you, please don't hesitate to contact us.