While you can usually tell what state gets to tax TPP by where the delivery occurred, the taxing of services isn't so easy.
1. Some states (and this is the default) simply tax the service if it is performed within the state.
2. Some states will tax based on where the customer receives the benefit of the services.
3. And some states will not tax if the result of the services (e.g. the item repaired) is shipped OUT of the state (related to number 2, but not precisely the same).
When these issues come up, you should assume option 1, by default, unless you can find something that says otherwise.
Sales Tax Guy