For the month end you are able to generate report on invoiced total sum.
Go to Reports > Financial > Invoices
Set the start and end dates for the report, usually first and last day of the month. Select the department you want to create the Report for. Click Generate Report.
The invoice report will show you the Total invoiced sum and list of invoices for the time period.
Note that consolidated invoices are not included in the report. The consolidated invoices are listed below the report and the original invoices are used in the report.
Related report: VAT Report