Demand that future payments be by certified check

Dear A review of your loan account indicates that you have had three checks returned unpaid from your bank. While we regret having to take this action, we must insist that all future payments on your loan be made with certified funds. Any payment which is not certified will not be accepted and will be returned to you, possibly causing you to incur a late charge. In making all future payments, please send them to the attention of (denote) for proper credit. Your cooperation is appreciated.