First select the name of your Organization on the top right of the page, then select "Reports" from the dropdown menu.
From there change the report from "Campaign Spend Report" to "Invoice"
Select the month and year you want the report to show, and select "Open".
On your invoice, you can see the amount of Blips received, amount spent, payments processed, and your current balance for the selected month.
This report is across your organization as a whole and does not specify spend per campaign or per billboard.