If a customer books an appointment service that you've assigned a retail product to, the assigned teacher will see a green product icon on the day of the appointment.
To add a product to a service, click Appointments > Services > edit a service via its dropdown menu.
Scroll to Products and click 'Add Product'.
Select the product and submit.
Note: you'll want to check that there are enough quanitites at the location where the appointments are taking place. You can do this by clicking Studio Setup > Retail Products selecting a Product and scrolling to the current quantities per location.