" If several reminder letters have been sent and there has been no payment made or reason communicated, it may be because the customer is sick or having financial difficulties. If even no partial payment is made after several reminders, a final reminder can be sent that clearly asks why the customer is not paying their bill. They can also be informed that serious consequences will occur if the payment is not made. The consequences may vary for different companies, but the most common are termination of the account, repossession of the goods or legal action."
From:
http://www.sample-resignation-letters.com/writing-a-payment-reminder-letter-with-samples.html
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