I'm all booked for 4 of us June this year and have made the following plans: Arrive Ullapool on a Friday, B & B and they'll keep the car. Ferry Saturday to Stornoway. Ride to Butt of Lewis on Sunday. HebShuttle on Monday to Barra. This is the killer bit! If it's Billy no Mates it's £270, if it's Billy 3 Mates (who don't need to be named on the booking form?) it's £270 shared 4 ways. We will then cycle back to Stornoway for the ferry to Ullapool and our cars.
I did look at cycling both ways when it was just me, thinking accommodation would be less than the shuttle fare. I also looked at driving onto Skye and getting ferries from there to save a bit on the return leg. First to can't remember to the south and back from somewhere further north. Saves about 60 miles?
Good luck, John