You can use any of the fields from the dni_invoices_get stored procedure. You would use @amount and @tax to get show the totals for the invoice. You could add fields to the stored procedure to return additional information to display. dni_invoices_get is used on the invoice edit page when you click the send email button. If you'd like to update the emails sent by the recurring tasks, look at dni_reminders_get_invoices. You can see this being used in the class SendInvoiceReminders.
To display the line item descriptions you would need to build the text in the stored procedure or look at modifying the code that does the replacing. SendInvoiceReminders.Run for the automatic reminders or admin/invoices/edit.aspx for manual reminders.
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Rob
DotNetInvoice Support