You can create separate discount rules via My Company > Products > More > Discount rules, which you can then link to the client. The Discount Rules are great for bulk invoices afterwards, where different customers have separate rates.
After creating a discount rule, it is possible to grant the fixed discount to the customer via Clients > Client > Discount rule.
Please note: Only applies when an employee assigns a product or creates an invoice for outstanding bookings. This discount rule is not applied when the customer purchases a product in the Shop.