A tip is generally not expected since rates are set based on what a provider is comfortable with receiving for her time and service. You booked a two hour massage session for 600, so I'm sure she worked the tip into that rate.  
   
 However, if you feel that she exceeded your expectations, feel free to drop an extra Benjamin and let her know that you will return. Repeat business is a great way of showing that you enjoy her sessions. Don't mention the tip - just leave it with the rest of the donation and let her discover it.