multi-user booking plugin

We are building a WP site for a tutoring company. There are many options that offer multiple tutors to customize their calendar of availability, users (students/parents) to login and book time slots and send automated email confirmations to both the student and tutors. What we are having difficulty finding is a plugin that will also count-down the number of hours left in a pre-paid package. For example, a parent will purchase a block of 20 hours of tutoring, and client wants the number of hours left to reduce by one hour each time they book an hour. If there’s nothing out there like this, can you recommend a plugin that is customizable and will allow us to manually code the functionality we’re looking for?