I use the ES site on a regular basis. Where possible it’s better to purchase via AMAZON.ES because their helpdesk is more akin to dealing with problems of this nature. Not that long ago La consejera de Hacienda del Gobierno de Canarias increased the threshold over which goods purchased over the internet must pass through aduanas. This limit is now 150 Euros. What you must do when you place an order with AMAZON.ES is put your DNI or NIE as the second line in the delivery address. You’ll still be charged IGIC but you won’t be charged IVA.
Some of this is described here:
Unfortunately, none of this solves the problem of an item being available on the UK site and not on the ES site.