If you're needing to check out a client that has Auto-Billing turned on in their profile for a set amount of monthly services, check out these steps:
Click on the appointment to open the Details page and select the "Checkout" button (the grocery cart icon in the top right)
Click the 3 vertical dots after the service name to expand the menu and select Apply Auto-Billing from there
You'll notice the remaining sessions listed there.
Continue to click through the prompts by entering in payment method to continue on through the gratuity process
Then you'll see the option to print or email a receipt