As Project customer advances/retainers generate project invoice proposal. If workflow and auto invoice emailing is active, the retainer/customer advance invoice would be emailed.
For many clients, the retainer/customer advance invoice is NOT emailed to the customer. Often the funds are received w/o an invoice, and the retainer/customer advance email is used for payment reconciliation. A simple toggle on the "Request a customer advance" form to enable/disable auto-invoice email would.