Why have the Correos charged me?

Hi, I would be grateful if someone could explain this please.

I’ve recently moved to the campo and ( as I have a birthday coming up) a friend has said that my family could send cards to her address.  My sister sent me a card costing £4 and paid £8+ for postage airmail track & trace! When card was delivered the postwoman charged a further 6€. I wasn’t there when it was delivered, so couldn’t ask the question . Can anyone tell me why I was charged please? Since Brexit I’ve been charged a tax on parcels being sent, but never for cards.