Wording follow up emails can be tricky sometimes. You don’t want to appear desperate, you don’t want to be pushy, but you sure as hell want to get paid.
I like to personalize all my communications, but I make an exception for payment reminders. In that case, I like the customer to know that they are in a system that will follow them up. That way, I don’t feel like I’m personally nagging, and they get the message that they need to pay now.