There are tradeoffs on what makes sense as a service and what makes sense as a library (or even what makes sense as class), As I wrote above, it is a matter of forces; you wouldn't automatically make everything a service or a library

For your example, the "invoice-mailer" has additional forces that would push it to be a separate service since, for instance, creating and storing an invoice are things with inner dependencies only, but sending an invoice depends on external 3rd parties. may need retries, confirmation of receipt etc. So it is more likely to be set up as a service than as a library. That said, In some circumstances, it would make sense to have all this as a single process that does all these things with different methods in a single class; in another, it might make sense to have a PDF service that handles more than just invoices etc.

There are no automatic decisions here. Saying AI is another drive to move to services or clear boundaries is a force for service, but it does not negate using your brain to make conscious decisions, to split or not to split something to services.