In the old preorder model, when customers use the partial payment method to place order with the pre-order item. The app has the function to send the reminder partial payment email to customers to remind them to complete the rest of the pay.
For your reference: https://globo.io/kb/how-can-i-remind-customers-to-complete-their-remaining-payment-when-they-have-partial-payment-for-pre-order-products