The service extras addon is fundamentally flawed in its design concept, the developers apparently didn't take into account the way such a tool would be used in the real world (Use Case).
Additionally, a core concept of software development and design is "Do not Repeat Yourself" or DRY and ultimately this concept applies to business practice as well. Imagine if you had to re-design and reproduce your business card every time you wanted to hand one to a potential client.
1) With the design of this addon, you have no choice but to do that, you have to redefine the 'Extra' for each service. You do not have the ability to define an extra one time and re-use it across multiple services. For example, using the demo concept that the developer provides if you offered 100 different services at your dental practice and wanted to provide the customer the ability to purchase a toothbrush as an extra for each service you would be forced to define the extra 100 different times. A tedious, laborious, and error-prone process; not to mention the technical overhead of essentially 100 duplicate records.
FIX: Allow for the creation of an extra a single time and provide the ability to reuse that extra on the appropriate services.
2) During the booking flow, the 'Extras' tab is displayed to the customer even if the service does not have any extras defined for it. This violates a cardinal rule of e-commerce; never put unneeded or unnecessary steps in the booking and purchase process. It will result in the loss of sales.
FIX: Do not display the 'Extras' tab when there aren't extras defined for the service.
3) When the customer has selected extras the customer is only shown the total price in the 'Cart' tab It's not obvious to the customer what extras they have added and will more than likely result in abandonment of the purchase.
FIX: On the 'Cart' tab display the service and extras as individual line items.