While we don't currently have a true invoicing tool, it's definitely something on our radar! We're currently working on researching how invoicing might work best for Schedulicity so that we can build that feature and make it as useful as possible for our businesses.
For now, we do have the Ticket Management feature, where you can email your client or print out a ticket of what they owe. It's definitely a handy way to notify clients of outstanding balances, though you'll want to keep in mind they won't be able to pay the balance using the ticket. Just a heads up there!
If you'd like to touch base with us on other potential workarounds or if you'd just like to chat through other payment questions, feel free to reach out to Schedulicity's Rockstars.