We've started insisting upon a receipt from Correo, then photographing the labelling, and the receipt, and writing to Correo for an explanation of the charges, and asking for a refund. Some people have had success with this ( if you believe the facebook groups).
We paid 10.95€ on a 20GBP gift sent to us in Spain from friends in the UK yesterday. I don't understand how the charges add up anymore?
We've paid out close to 100€ in post/tax charges since our daughter was born, to the point where we've told people not to send anything. Insane.