A lot of booking systems have a point of sale POS built in. So when you finish the service, you click the appointment and it takes you to a check out, where you then take payment from the client, and offer them a receipt (paper or by email).
A POS also means you have financial reports that you or your accountant use for book-keeping, accounts, tax returns etc.
I couldn’t see if slick has this or not? Or whether that’s important to you or not.