All visible text fields are inline editable. Most common way to add your company’s tax number is to write it on the upper right corner.
Step 1: Please navigate to the Apps section of your Shopify store and click on Simple Invoice.
Step 2: Click on “Dashboard” from the upper left corner and click on “Customize Templates”.
Step 3: You’ll see your company information on the upper right corner. Click on the last line and press the “Enter” key. Now type your tax number and save your template.