Delivery charges for goods purchased online to addresses in the Highlands and Islands has been a big problem for years. There are two issues which contribute to the problem.
The first issue is that sellers have been glib in the way they describe and promote their delivery charges. Often sellers would describe their goods as having "free UK delivery" or "free delivery on all orders" and it wasn't until you got three-quarters of the way through the purchase process when you discover that phrase didn't apply to the Highlands & Islands. Having just wasted your time with one seller you would try an alternative only to discover the same issue. You could easily go through this loop several times.
In April this year (2018) the ASA made it clear that if a seller states "free UK delivery" (or similar phrases) it should mean exactly that and it is unacceptable to then have exclusions. If you come across a seller making untrue claims about free delivery you can complain to the ASA or contact your local trading standards office to make a complaint.
Kevin Costner as The Postman (1997)
The second issue is that some sellers just aren't interested in finding cheaper delivery services for their prospective customers who live in the Highlands & Islands. They choose a courier that offers cheap delivery for most of the UK, but not for the Highlands & Islands. This is an entirely legal business decision, which the sellers are entitled to make, just don't expect me to purchase anything from them.
Online retailers could offer delivery via Royal Mail which charges the same prices regardless of the destination. Royal Mail are less competitive for heavier or bulky items but offer good value for small parcels up to 2Kg. For larger parcels there are low-cost delivery companies, like My Hermes, who offer competitive prices for parcels going to the Highlands & Islands. If companies want my business they just need to put a little effort into getting reasonable delivery prices.
The screenshot below is the delivery charges page (taken on 12/06/2018) for a company called Site King. Site King sell workwear and personal protection equipment from their own website. The goods offered were attractive and were competitively priced, except when it came to the delivery charges for the Highlands & Islands. Their FAQ page suggests that they have a minimum delivery charge of £13.33 plus VAT (£16 incl VAT) for the Highlands & Islands.
This UK based company is charging more for delivery to the Highlands & Islands than for any of the following countries: Austria, Belgium, Switzerland, Cyprus, Germany, Denmark, Spain, Estonia, France, Hungary, Italy, Lithuania, Luxembourg, Latvia, Malta, Netherlands, Poland, Slovakia, Slovenia and Sweden.
I know that it is possible for Site King to offer alternative delivery options because I first came across them on Ebay when I purchased a pair of men's cargo trousers. On Ebay, Site King list their products with free delivery (including the Highlands & Islands) and, in my case, the order was delivered by Royal Mail. Many other ecommerce websites allow the customer to select a delivery method, and therefore price, and I'm sure it would be possible for Site King to do the same.
I would prefer to purchase from a company's own website but I am not willing to pay a £16 surcharge for the privilege.