The solution I came up with is to adjust the due date on all invoices I do not want reminders to go out on, to reflect their payment plan. That is, if the invoices are scheduled to be paid off in 3 months, I'll adjust the due date of invoices involved until after that date. This way the customer does not get reminders. However if there is a hitch in their payment plan, I believe they'll start getting reminders after the revised due date.