In your Billing Settings, you can indicate when an invoice is "Past Due." This will help you stay up to date with your billing and identify which invoices most need your attention.
To set this up, go to My Account > Settings > Billing and Services > Settings and choose the number of days that would trigger a past-due designation on an invoice.
You can see here that once 30 days have passed since the manual or auto-generation of that invoice, the status will change to "Past Due."
If you wish, you can choose to have "Past Due" invoices trigger an automated email to your clients. Many practices use this option to collect payments through the client portal.
If you'd like to send your clients "Past Due" emails, just select the option: "Email clients when an invoice is past due. Only for 'client portal' and 'E-mail' delivery methods."
You can preview and customize the email clients receive when this option is selected by going to Settings > Client Portal > Invoice Past Due
Once a payment is applied to a "Past Due" invoice, it will update to "Paid."