A daily server job checks all Signed and Approved and Partially Paid proposals whose Due Date + Net Days is earlier than today and sets them to Past Due. For each one:

• If Auto Charge On Due Date is on, Sprwt tries to charge the balance to the customer's saved card (success → Fully Paid; failure → auto-charge error flag and emails).
• Otherwise it emails the customer the payment past due message with the remaining balance and a link to pay, emails your kitchen address, and notifies proposal team members.

The customer can still pay from their proposal page while it is Past Due; a payment moves it to Partially Paid or Fully Paid.