You can add extra details to your account invoices for taxation purposes. To do so:

1. Click your profile image at the top right, then click Account settings.
2. Click Company details.
3. Below "Company details", click Change details.
4. In the "Invoice details" field, enter the details you want to add to your invoice. 
   
5. Click Save changes.

The details will show at the top of your invoice, as shown in the image below.