Dear

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 us at any time