I purchased online 3 small items from UK retailers to be delivered to family in UK. They posted the padded bag with a declaration in early January. Value of goods included was €31, including 3 engraved metal plates. After a couple of months I get a notification from Correos asking for details of the contents. I sent scans of the purchase documents. More emails back and forth, until I told them that this was not a commercial purchase, nor samples, but private purchases and so not worthy of all the bureaucracy. Next is a demand to pay €6 duty for the plates, plus a €20 service fee, plus IVA and some other additions, bringing the total to €38. For a package worth €31; so a 117% surcharge. I paid just to end the nonsense. A week later the package arrives, gets signed for and I find that the engraved metal plates have been removed (i.e. stolen). So I paid duty and fee and other surcharges for absolutely nothing at all. It will be pointless in trying to complain or to recover the plates or get a refund, so this post is the venting of my anger at Correos.
The moral of the story NEVER have anything of any value whatsoever posted from UK to Spain.
Posting anything from UK to Spain will force Correos to become super-efficient in extracting every last centimos of revenue out of your package.
And for all the Remoaners still not over it; don't bother using this event as an example of Brexit consequences; we get it.