You'd like to view invoices for your Enterprise Team, but the Billing tab for your team does not show any of your invoices.
Currently, Heroku Enterprise Customers are unable to see their monthly invoices, as they do not receive an invoice in the same way that customers that pay Heroku directly every month do.
There is currently a private beta, one feature of which allows customers to download a CSV of monthly usage, but it is not yet widely available. There will be much fanfare when the new style of Enterprise Accounts and Enterprise Teams goes to public beta, and team usage will be much more widely available at that time. In the interim, we recommend you ask your Heroku Account Executive to pull a usage report for you.