Add-Ons should be simple to manage. Currently you need to add an add-on then assign the add-on to a calendar one at a time by clicking edit service and then add the add-on.

Easier method would be to manage the add-on to assign it to which services available direct inside of the add-on menu with a simple toggle button to assign the add-on to the services available.

Similar to the interface used with assign calendars to a group.

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Create add-on

assign add-on to service calendars (select all or select services)

save and add-on is added to the service.