When you access the help center there is a tab at the top of the page labeled Sales Invoices. Clicking this tab will take you to a page where you can access your current and historical sales invoices. By default the sales invoice home page will only show your recently viewed invoices. In order to see all of your invoices please click "Go!" at the top of the page.
The refreshed page will show you all of your invoices, including any new invoices. Click on any invoice name to view the details of the invoice or make a payment.
If the Sales Invoices tab is not available to you or you need more information or additional assistance please contact customer support.