Just a comment that when we stayed at a (very large) hotel in Sorrento, we figured their land-partner would have a huge incentive to please the hotel's customers. So we booked with the concierge, and the car was charged to our hotel account. I will say that the land-tourism company was so large that our first driver told us that we could not "ask for him" for our transfer to the Naples train station at the end of our stay.
I would observe that it might be MUCH cheaper to use a Positano or Amalfi company for the inter-hotel transfer. Your driver might have to spend two hours with an empty car if he comes from (for example) big, busy Sorrento or Salerno to drive you between Positano and Amalfi.
It's wise to ascertain from each hotel just how close an auto can get to their property, in those towns! I hear some hotels in Positano dispatch Hill Porters.