Because Positano has, liternally, hundreds of stairs, it may be wise to be certain of the location of your hotel, and how luggage usually gets to the hotel. Oh wait, you are in an AirBnb and maybe they won't tell you where your accomodation is until the last minute? One of the issues with the Positano ferry is that the (recently renovated) dock takes only smaller boats than some of the larger ports, like Sorrento. There is no magic solution to this, and June will be, as usual, very crowded in both Sorrento and Positano.
Another issue is that a car service (which I also endorse) may or may not be able to get "close" to your AirBnb. Positano has only a few roads, and is mainly a steep, vertical, pedestrianized town. Note that 4 passengers is a full load for a sedan. But will your luggage fit in, say, a Mercedes trunk?
You may wish to price car services from the Naples train station. The Circumvesuviana is so, so crowded and so hot that it may be worth a small premium to avoid it. We took the elbow-to-elbow Circumvesuviana for our arrival in Sorrento, but were happy to pay the hotel's land-services partner 100 Euros (years ago!) for a car back to the Naples train station! Helping our decision was an excessive (if not unreasonable in the markeplace) taxi fare from the Sorrento Circumvesuviana station to the Sorrento Hotel, about 20 Euros. That made the car service fee seem more reasonable!
Make sure you have good plans for your departure. For example, you cannot fly home from Rome on a day that you wake up in Positano.
