It all comes down to logistics and what the punter will or should pay.
I have a number of items from Hong Kong and that region on a near monthly basis via ebay purchases. The price including postage is far cheaper than the equivalent or similar item from local suppliers
The same applies with items from Amazon UK and USA. Same postage, but shorter UK travel differences.
The only hiccup is the waiting time, unless you can wait the 5 to 21 days delivery time from abroad?.
It isn't just the Scottish Highlands that suffered the monopoly effect. Ireland, both North and South, had similar problems with excessive price from and to mainland UK. That now seems to have stabalised over recent times.
Don't forget, that some people add higher delivery charges to cover packing and the lower advertised price of the item!.
Another thing worth considering is 'free delivery'. In the case of Amazon, on certain item 'forfilled' by them are delivered for free. The same applies to some other retailers, with possibly a spend of over £5.00 on an item, with the result of no delivery charge. Why pay £4.99 for one item and perhaps £10.00 delivery, when you can buy two items for less?.
Phew, glad that I got that lot of my chest!.