Based on our assessment, the Dunning Letters seems to be an overkill for us. Our goal is to basically send the list of invoices that are days past overdue for certain days per customer in a single email through a workflow. Is this possible? I tried a scheduled WF with a transaction saved search but it sends the overdue invoices separately. If it's based on a customer saved search, it sends the entire customer statement and not only those that are included in the search.

At my previous employer I wrote a workflow action script that would attach all open invoices to a single email they used for overdue reminders.

That's beyond my skillset. Have you tried just including the invoice numbers of the overdue invoices in the email body of the Send Email action of the WF?