It is difficult and expensive to re-engage the customer with sales information after they have made the service appointment. Give them the offer while they are making the appointment.
These leads are from people who have agreed to speak to a salesperson while they are at the dealership- or before they arrive. Almost always, these are the primary lead source- not a secondary lead from the same car shopper. Even better, they are already your customer, and you know where to find them- on the service drive at their appointment time.