For adding the Tax ID Number, I would normally edit the “Footer Details” content of Custom Form Styles.
Follow the following steps:
From the home screen, click on the Gear Menu. Beneath the column heading “Your Company or Settings,” select the Custom Form Styles tab. On the Custom form styles page, select Invoice default (or standard) form type and select “Edit” button Action column. Switch to the “Content” tab. On the right panel, click the 3rd bottom “Pencil” icon at the far right.
On the left panel, you will notice two open fields:
1. Message to customer on
2. Add footer text
Choose the one works best for your business. I prefer to use “Add footer text” field.
Be sure to save your changes when you’re finished.