You can download invoices for all completed orders as a customer within the My purchases section in your Legiit account.
You will filter your orders by a custom date range up to 90 days and be able to download invoices in bulk to your computer or device.
After you've selected your custom date range, click the "Bulk Download Invoices" button on the top right of your purchases page: https://legiit.com/buyer/my-purchases (pictured below)