The customer can book their order and take a turn. Two days before their turn, they receive a notification to see the shop's available offers so they can book something.
Each barber can only see their own customers, and any time changes affect all their customers.
The administrator can see all barbers and their customers, set prices, post offers, and track income and expenses for any period. They can see how much each barber charged for a haircut and the total revenue earned during any given period.
The customer books and receives a notification, the barber manages his clients, and the admin monitors everyone.