I generate a email like the following.
Please do not reply to this email as it is automatically generated by our window cleaning softwareYou have received this email as your account has fallen outside our credit terms.
To avoid a late payment charge, please make immediate payment.
You can pay by bank transfer using the following information.
Account name : The big fat one.
Account no : xxxxx xxxxx 666
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I find doing it this way, if the customer takes offense. You can say that when any account falls out side our credit terms, a email is automatically generated and sent.
In other words the computer has done it
Roy