If your customer replies to an email or SMS reminder, the reply will show up in the Messaging tab. You can also reply directly back to them on the same page. Any future messages in that thread will be sent between you and your customer.
Additionally, you can get a text or email notification when a customer replies to a reminder message. To enable this:
Go to the Reminder Settings page.
Click the dropdown menu under "Notifications" and select either "Text me" or "Email me."
Click Save.