Single customer payment reminder for all the unpaid invoices and not a reminder for each single one.
If a customer have 10 unpaid invoices receive 10 single reminders, 1 for each invoice.
It's much better to send a single reminder including the list of unpaid invoices and the statement or at least to attach the list of unpaid invoices and the statement to each single reminder.
Gina Leserra commented
I too would like to not infiltrate my customers inbox with individual reminders