I suspect that one of the issues is that short stretch of water that costs so much to cross by ferry. It takes at least an hour each way including check in and loading time and then prob at least 45 mins each way from the ferry to Sandown. Add in say another 45 mins for unloading and you have a minimum of 4 Hrs 15 plus ferry costs to make a single delivery, not counting the time to get to and from the mainland ferry port. That's a significant overhead that would push the cost up compared with mainland costs and who would uplift fuel if it was cheaper on the mainland?