To 'redownload' Invoices that you have already downloaded do the following:
1) Look for an invoice that is marked as 'downloaded' and click on it to view it.
2) On the Invoice page click on 'edit header' link at the top of the page.
3) Search for the field 'Downloaded'. You will notice that a downloaded invoice is marked as 'T' for true. If you want to unmark it so you can download it again, enter 'F' in this field (F for False).
4) You can now download this invoice again.