There are two ways to offer discounts to the customers:
Specify a fixed discount for a customer:
A fixed discount for the customer can be specified in their profile in the additional tab. This will apply the fixed discount to the Invoice total.
Custom Pricing for an Service:
You can choose to charge a custom price for a service to a customer. For instance, if you want to offer a discount to the customer or add a surcharge to the service you can change the default price for an appointment.
Custom price can be set when an appointment is created or by editing an appointment (single appointment or series).
Customer price can also be updated while generating the single invoices.
Custom Discount Lines:
When generating a single Invoice, you can add custom lines to the Invoice and they can be set to be either discount or surcharge.