You can access your Billing History from within your account. Go to My Account and click on the Usage tab. The top section displays your current usage of emails and bytes. The two tables below show your Email Usage History and your Billing History. Your Billing History will show a complete breakdown of your historical credit card charges that you can review. PDF icons on the right hand side allow you to easily download a copy of your receipt.
If you currently pay by check, please contact Support for your invoice.