Payment is required from you when you book the Session because the time is being held specifically for you, at your request! That time is now allocated to spend it on you. Payment is required in advance as part of a reasonable and prudent business practice. Granted, it would be wonderful if we lived in a world where there was complete trust and payment could be made after the Session. But it isn't a perfect world and you shouldn't want to have a Session with a professional who didn't himself have in place good business procedures and policies.