It would be interesting if you gonna remind a customer automatically if delivery is delayed. Let's imagine:
Order status has not been changed to "Delivered" within 7 days => Change order status to "Delayed" (for sure you have to create this order status with email templates) => It would automatically send an email to customer: "Sorry for the delay and inconvinience, we are still processing you order. Here is a *promocode* for your next order".