We've added an oft-requested feature to AWS. You can now download printable, PDF-format invoices from the AWS Portal. Invoices are available for the month that has just passed (I can access my August invoice now) and for all prior months.
You can find the invoices in the Account Activity section of the AWS Portal. Scroll down to the bottom of the page and look for the section labeled Previous Month's Activity:
Select the desired month from the menu and click on Go. You'll see the following summary near the bottom of the page:
Click on the PDF icon to download the invoice: