Enter the Billing page and pay attention to the topmost box. The first long box at the top is usually "Remaining Package" (Today's message quota). You can see a color graph showing the remaining message percentage.
The middle part of the screen displays a column of "Bills" history from past months (Invoices List) with a small Paper picture icon to download the PDF receipt proof.
If the bill changes its letter color to red "Unpaid", you are asked to click on that one bill which will then open the Card / Account Payment button screen.