If a developer is available to help you, they will reply to this ticket, but, we are not able to take on custom work at this time. Please take a look at the documentation referenced above and see if it contains the information you need.
Do you have a test site where you can recreate the problem?
Please provide a link to a page where we can see the problem. Create a private reply with admin credentials so we can take a look in the backend if necessary.
Please make sure that all WP Invoice plugins are up to date.