Create a field that totals up dollar amounts from invoices that are assigned to a specific agency

If you want the total on the 'Agency' record you can create a summary field to pull the total from all the individual 'invoices'.  You can set different criteria for only summarizing data that is 'outstanding' or any other defining fields you have on the invoices.



If you want that total to be on the invoices related to the 'agency' you can then pass that lookup, back down to the 'invoices'.

You may want to make defined summaries, because the situations may vary depending on the 'status' of those invoices.

Then wherever you make your email, you can have those specific fields.