How Do You Handle Unpaid Clients?

If you're a business/agency owner, how do you track which clients have or haven't paid? Is it through payment gateways (like Stripe, PayPal) synced with your CRM or accounting software? I’m developing a system that automatically checks the client payment status and sends reminder emails. Does your CRM or any software you use show payment status directly, and how do you automate payment reminders effectively? Would love to hear your methods for streamlining the process!