When clients are leaving your office after an appointment or if they're calling to ask a question, a habit that you may want to get into is checking to see whether they have any Open, outstanding invoices that need to be paid.
The easiest way to do this is to navigate to the client's profile and click on the Payments & Invoices tab. On that page on the client's profile, you can see that the second table is a list of all of their Invoices / Quotes -
In this table, you can click on the View button next to an invoice to open that invoice's detail page -
You can also check the box to the left of the invoice and then click on the red Delete Invoices / Quotes button. Be careful while doing this, as it will delete all the line items on the invoice as well and an invoice that has been deleted cannot be recovered -