Please log in to the FlightStats Developer Center:
Then click on Account | Invoices
You can download a PDF version of your recent invoices.
Note: Invoices are generated on the fifth of the month for previous month's usage. An invoice will not be generated if you had very little or no usage. Invoices are only sent to the admins on the account for Commercial Plan customers. A workaround would be to set up a Delivery List email address as the admin user (e.g. firstname.lastname@example.org).
For Contract Plan customers, invoices are currently not available in your Developer Center account. Your invoices are auto-generated and emailed to your billing contact at the beginning of every month. If they have chosen to pay by card, the card that you've added to your Developer Center account will be automatically charged on the date the invoice is due.