In WriteUpp you can add your custom fields to your invoices. Just follow the steps below!
Create your custom fields that will appear in the patient summary (if you haven't already done so) -> How do I create my own fields in the patient summary?
Go to Settings -> Invoice and click on the Layout tab
Select which custom fields you would like to appear on your invoice. You can add up to 3
Click Save
Your custom fields will appear on your invoice here:

Create your custom fields that will appear in the patient summary (if you haven't already done so) -> How do I create my own fields in the patient summary?
Go to Settings -> Invoice and click on the Layout tab
Select which custom fields you would like to appear on your invoice. You can add up to 3
Click Save
Your custom fields will appear on your invoice here:

Updated on: 22 / 01 / 2021