Hello vppoulis Thank you for using ITFlow.
I'm guessing you are in Europe where you have a VAT Tax ID assigned to each client that you invoice, is that correct?
Changes that you make to the code will be overwritten upon update and is not recommended.
We will be supporting custom fields in the near future but not at the moment.
What you could do for now is use the Client Notes to put in extra data like Tax ID.
You can also use the Invoice Notes section to include their Tax ID: and configure the invoice footer to include your Tax ID.
Not Ideal but will work for the time being.