The Nudgify billing works per Organization. This means that your invoices are organized per Organization as well. Read more about managing multiple Organizations in your Nudgify account.
You can view you invoices of each Organization you are the owner of via your Sites & Organizations page.
From you Sites & Organizations page, click the settings icon of the Organization you want to view your invoices of.
On the Settings page or your Organization, you can find the invoices in the Invoices tab.