Lol, stop digging that hole, Northern Ireland is part of Ireland, the clue is in the name.
LOL. The point is it doesn't cost more to send goods to Northern Ireland if Royal mail was used. I could get the ferry over for less £10 on a Stenna line daytrip and pick it up myself rather than pay over £14. Your stupidity over Northern Ireland also beggar's belief - It is a country of the United Kingdom, not just a political entity. You do realize that South America is a different place to North America as using your logic you need to brush up on your Geography. Thought you would have noticed how the Northern Irish politicians are sitting over in Westminster calling the shots.
If you weren't so thick you might have noticed that that the Irish sea is between the UK mainland and Ireland, thats why it costs more to send goods there. Northern Ireland is simply a political entity, geographically it's all Ireland. It's hardly rocket science.
LOL mate, clearly you should drop the r out of your username if you don't know Northern Ireland is part of the UK. aLV426 is also correct, the Royal mail are under obligation to service the entire UK at the same price (assuming this retailer uses RM).
My XL also arrived today, very please, looks and feels like quality, can't go wrong for 16 squid. Cheers OP