Invoice overdue email subject

This is the subject of the email that is sent when an invoice is overdue. Available variables are:

• <TMPL_VAR NAME=INVOICE_NUMBER> the number of the invoice that is overdue
• <TMPL_VAR NAME=PRODUCT_NAME> the name of the Product in System -> Configuration or in custom API
• <TMPL_VAR NAME=SERVICE_EMAIL> email of the person set as contact in the customer
• <TMPL_VAR NAME=XXX> Name of any column xxx from table 'customers', referring to the customer that is invoiced
• <TMPL_VAR NAME=YYY> Name of any column yyy from table 'people', referring to the person that is set as contact for the customer that is invoiced