We are travelling to Santorini and Naxos in May. Both hotels have a customized breakfast delivered to the room daily, and have arranged airport / ferry transfers (payable to the hotel, not the driver). What is an appropriate cash tip for each of these services?
Tipping is not part of the culture.
We were just on Santorini and I was uncertain about this as well. We did not tip the driver who brought us to the hotel from the ferry (included with the hotel). Meanwhile, I sent a message to a Greek friend in Athens who said 5 euros would be appropriate, so we tipped that on our return trip off the island. I also asked about tipping at our hotel. They were very service oriented, and my friend said 10 Euros would be appropriate for a hotel stay that was satisfying. We left a bit more than that. I'll say that we did not tip at hotels anywhere else on the trip, only here because it felt like the circumstances were particular.
Tipping is indeed part of Greek culture, but it's considered rude to ask for it.
No tip is ever required, it's up to you to decide whether or not to tip, and how much.
Taxi booked by hotels are generally overpriced compared to booking a taxi by yourself at the standard rate (which is already very expensive, especially in Santorini). So I would never tip the hotel staff for booking me a taxi.
If the price of breakfast includes room delivery, and if it's "customized" with an additional charge, this charge will be added to the final bill. I don't see any reason to add a tip to the service itself. However, at the end of your stay, you can tip the person who brought it to you every morning if it's always the same person and that person was very friendly and helpful.