How to let your customers know that they will be charged for their invoice the following day?

For this case we have a setting in Settings -> Invoices & Collections -> under "Due Reminders -> Invoice Due Reminder" -> where you can toggle ON and the day before the invoice is due Pencil will send a reminder to let them know their invoice will be processed the following day. 

You may want to check this article for more information:

How do I notify my customer the day before Pencil will take payment from their nominated payment method?