I have [refunded / free upgrade / given some other bonus] your account. We will also make it a priority to [describe how we will improve] to make sure this doesn't happen to anyone else.
Please let me know if there is anything else we can do to make this right or if you have something else to tell us. You can contact me by email me@example.com or phone $phone_number. Those are direct to me personally, not to our helpdesk.
Best regards
$me