How to trigger notification to invoice creator

Requirement: How to send FYI notification to Invoice creator once the invoice is approved/ rejected in all cases. IN other words, system is currently sending notification to user who initiates the Invoice approval, but business wants to trigger notification to the user who created the invoice.

There are multiple stages and after completing all stage (approved/rejected), notification must be send to invoice creator. Also invoice approval initiator should not get notification either invoice is rejected or approved.