1. Create the extra custom field you need via “Products Settings”. For detailed steps, see the guide on how to create custom product fields.
2. From the Templates dropdown, go to Printable Templates.
3. Click Invoice/Estimate Layout.
4. Choose the template you want to add the field to.
5. Select Item Columns.
6. In the Field section, choose the custom field you created earlier.
7. Click Save.
8. When you create an invoice, select the template you modified, the custom field will appear as a column in the item lines. For more details, refer to the How to Create a Sales Invoice guide.