If she starts your session 30 minutes late, she should offer to tack on an extra 30 minutes at the end to give you your full time or compensate you in some other way. She can't just shortchange you like that and expect you to pay for a full session. What did she say when you handed her the check or cash? If it were me, I would have handed her only half the amount. She owes you. I know it's not just the money, but she could at least make good on that part of it. And acknowledge her lateness and overcharging instead of just not mentioning it.