You can view your invoices by selecting 'Order History' from your Account drop down menu. You will need to login to your account to see this:
You can view your invoice for any new or past orders by clicking 'View':
Once you are on your order, to display the invoice, click the 'Invoices' button in the top right:
To print, click 'Print Invoice', choose your printer from the menu and click 'Print'.
To save as a PDF, click 'Print Invoice' and select the 'Save as PDF' option in the Print window (Windows) or select the 'PDF' option in the lower right of the Print window (Mac OS) and click 'Save'.