Reply to Notice of Item Missing from Carton
Your letter informing us that the [missing item] for your new [product] was not included in the carton has been brought to my attention.
There is a package on its way to you now that includes the item and a little something extra. This is our way of saying how sorry we are for having slipped up, and thanking you for choosing the [name of product].
If I can be of assistance to you in any way in the future, please call at any time.