Dear (recipient's name),
We have sent several notices letting you know your account is in jeopardy of closing. We don't want to cancel your credit and hope you'll get your account current as soon as possible.
We are sure the matter of non-payment is an oversight on your part. You have been such an excellent customer in the past. Please give us a call at your earliest convenience so we can help you get your account current.
Sincerely,
(your name)