At Appoint, we have Add-on feature which allows you to sell products at the time of checkout. Each service can have its own add-on. An example of Add-on can be, a tour company can ask customers to include meal or a Salon can sell a product.
You can set add-on for the services from the service area of your admin console (Settings => Customize => Click on image icon => Select Add-on => Add New Add-on).
Note : Its a premium feature part of the PRO and above membership.