Courier services to the Islands can be a bit slow (they actually hand it over to a local firm in Inverness, who then kick the parcel around their depo yard for a day or two before delivery). This two-stage stuff pushes the price up.
Royal Mail is fast, efficient and doesn't cost any more than any address on mainland UK.
Sensible companies, like Spa cycles, send small stuff Royal Mail.
Wiggle/chainreaction/et al always use a courier firm. So a small package, that would get here in 2days standard Royal mail parcel post, takes over a week via Herpes/DPU etc.
This is what drives people to use Amazon. They fly their stuff in and orders are delivered in 24hrs.