Hi Jay , Celine from the Community Support team here!
You're right, once a team switches to the Starter plan, the billing section is no longer visible in the Figma UI. That said, we can still help retrieve past invoices for you.
To move forward, please reach out to our support team directly via this form using the email associated with your Figma account: Submit a request. (Just mention you’re requesting past invoices, and the team will be able to assist directly.)
Hope this clarifies, let me know if you need help with that!