Invoice email subject

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

• <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=PRICE> invoice amount
• <TMPL_VAR NAME=NEW_BALANCE> new balance of the customer
• <TMPL_VAR NAME=CREDIT> new credit of the customer
• <TMPL_VAR NAME=CREDIT_WARNING> Boolean, takes the value of 1 if the available balance of the invoiced customer is equal or below 0
• <TMPL_VAR NAME=DIRECT_DEBIT> Boolean, takes the value of 1 if the payment type of the invoiced customer is “Direct debit”
• <TMPL_VAR NAME=CURRENCY_SYMBOL> Currency symbol, according to the currency of the invoice
• <TMPL_VAR NAME=IS_PAID> Boolean, takes the value of 1 if the status of the invoice is paid
• <TMPL_VAR NAME=START_DATE> The start date of the invoice in human-readable format
• <TMPL_VAR NAME=END_DATE> The end date of the invoice in human-readable format
• <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
• <TMPL_VAR NAME=ZZZ> Name of any column zzz from table 'invoices', referring to the current invoice