Yes, you can do this yourself via your personal portal, without needing to get in touch.
How does this work?
Log in to your personal account (app.fellos.nl).
Go to "View treatment" and scroll down to "Your subscription".
Click "Adjust quantities".
Use + or − to choose the number of pens for your next order and click "Continue".
Briefly explain why you need more pens (for example, travel or extra stock).