The second payment is something the guest is told about several times during the booking process and should also be in your renter agreement. However, it can still surprise the guest when it happens and an email warning is often helpful.
To do that, create an email template in OwnerRez with the below subject and body, and then create a trigger to send it [x] days before arrival. Make [x] days equal to the number of days before arrival that the second payment is due plus the number of days warning you want to give. For instance, if your second payments are collected 30 days before arrival, set the trigger to be sent 35 days before arrival so that the guest gets the email 5 days before the payment hits.
Subject:
Second payment reminder for {PDISPNAME}
Body:
Hi {CFIRST},
This is a reminder that the balance of {BTOWE} will be collected on {BSPDATE} for Reservation # {BID} for your stay at {PDISPNAME}.
The payment will be charged to the card used {BCRE} to pay the deposit for the reservation.
If you are satisfied with that card for payment, there is nothing you need to do.
If you want to change that card, use this link before the due date:
{BUPAY}
When payment transaction is complete, you'll receive messages with the directions and details you need to enter the property.
Thank you for making {PDISPNAME} your choice.
{MYFIRST}
{MYCO}
For other examples of email Templates, check out the Email Template Library - Overview.