The Service Add Ons feature is not thought through to its end.
Service Add-Ons are services and services need providers. So it must be possible that Service-Add-Ons are connected to providers.
Service XY is executed by Provider A and by Provider B.
Service add-on ZY is executed by provider B.
Now the customer selects Service XY and the Service Add On ZY.
Only Provider B can execute this service.
A real life example:
Service Haircut is offered by Provider A and Provider B.
A service Add On Hair coloring is offered only by Provider B.
The customer chooses Hair Cut with Hair Dye. The whole service can be performed only by Provider B.
Please be sure to connect to providers. Other systems can already cover this.
Service addon is a service.