Awful customer service. I had the unfortunate event of arriving late to my prepaid treatment and I was heavily punished for it. So if you normally have a busy life this might help you.
1. Avoid prepaying online. You are only risking they keep your money for a much cheaper service (or none).
2. If you already paid online, accept whatever they tell you and go to the treatment room. Do not put your clothes back on until you receive what you paid for because they will not refund anything.