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).
Ordered cork underlay from Engeland.
Get an email: You are in NI it has to come from Poland.
Me: That's ok.
Now get another email: And it's 20% vat on top now.
Me: WTF
I asked why as NI is in the EU customs union but if this goes on might have to try to source elsewhere.
Honestly...! Edit: I'm just going to ring them maybe it's just confusion cos emails.