Some Amazon items are free (slowest) delivery even if under the £20/25 limit and not a Prime member - I presume the seller takes the carriage cost..I went to order something this afternoon for 22.99 and it said I needed to spend another 2.01 for free delivery. I checked the T&CS and that said £25 was the limit.
I found a market trader there that had a slightly better product for 98p more and free delivery.
My Amazon account now states free delivery over £25 (or £10 books) so it's not a Skye, Islands or Highlands thing.