Automatic Dunning Emails - Per an invoice, with invoice attached as PDF

Hello - I am in the process of automating our collections process and so far I figured out how to send account statements on the customer level. The problem is most of our customers have different AP contacts for each project. So what I would like to do is get netsuite to send emails for each invoice that comes past due and also includes the PDF attachment of that invoice.

When building the customer statement process, the first step for me is to create a custom checkbox field on the customer or project level that acts as a flag for all customers/projects that I want to be included as an email recipient. This acts as my reference point for when I build a custom saved search that will be used for my "Send Email" workflow. I was hoping I could do this on the invoice level but I do not see invoice as an option.