Your invoices are always available for you via the Marlins Online Test Platform (www.marlinstests.com).
Simply log in and select "Purchase History Report" under the 'Reports' tab at the top of the page (Direct link: http://www.marlinstests.com/admin_report_purchase_history.php).
Once you have selected the dates for the period of transactions you wish to view, you will see the option "View Invoice" next to each line.