Personal Trainer Customer Booking System

General information for the business: Personal Training Description of requirements/functionality: Hi I have a client requesting a booking software. This client is a personal trainer, he sells a package which allows his customers 16 sessions for €120. However he wants his customers to be able to book multiple sessions at a time, also be unable to book more than 16. When they have used up thier 16 sessions they will be unable to book anymore until they pay him in person, not online. Once they pay him he can then go on and give them 16 more bookings to use. Does a plugin exist that can perform this or can you develop software that I can integrate into the wordpress website? Extra notes: