This is their last chance. Prior to this you may want to try a phone call or telegram or other attention getting device. Give them one last brief period to respond before taking action.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
As all of our previous attempts to contact you were unsuccessful, I regret that we are forced to <consequences of nonpayment>.
In order to avoid this unpleasant action and further damage to your credit record, please pay the balance of your account, <$amount>, within <deadline> days of the date of this letter. This is the last notice you will receive before action is taken. Please give this matter immediate attention.
encl: <List of enclosed items goes here>