How do I email invoices with specific naming convention?

You can do so by adding smart fields into your emails. Adding smart fields gives you the flexibility to customize the subject, file name, and standard messages of your emails with actual names and IDs. This results in personalization of communication. These settings can be overridden at the user level.

Smart fields are placeholder fields (variables) that allow CORE to auto-fill them with IDs or values when emailing your invoices, statements, reports, POs or receipts. You can add multiple smart fields to the subject, file name and default message of your emails. For example, you can add {ClientID}{ProjectID}{Invoice_Number} to the Subject field of invoices. CORE auto-fills it with the relevant client, project and invoice number when composing the email. E.g., COTTorrance Airport:1222 where COT is the client ID (City of Torrance) and Torrance Airport is the project ID and 1222 is the current invoice number.