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.