It is critical that subject lines for emails are both effective at getting attention and also provide information to help the person receiving the email take action and pay.
Key components of a subject line:
Who is sending (this is usually taken care of by the sender of the email, but if not this needs to be addressed)
The call to action “Payment of XXX Needed”, “Payment of XXX Required”, “Invoice XXX”
The Invoice # (to reference)
The Purchase Order or Authorizer (reference)
Examples:
Payment of $343.27 Needed for Invoice 12345 on PO 98765
Payment of $450 Required for Invoice 12345 on PO 82311
Invoice 12345 for $343.27 on PO 98765
PAST DUE: Payment of $343.27 Needed for Invoice 12345 on PO 98765