A dunning letter is a notification sent to a customer, stating that the customer is overdue in paying an account receivable to the sender. Dunning letters typically follow a progression from polite reminders to more strident demands for payment, if the customer continues to be non-responsive in paying. The first few letters that are sent to a customer should be polite, on the theory that the customer has simply overlooked payment, and the company wants to retain its goodwill for future business.

As it is mentioned in the definition, its a notification. So, اخطاریه, can be suitable term.