Last year I had the tee-shirt sent to a UK address.
Looks like the despatch from Northern Ireland doesn't necessarily mean that EU import charges are avoided. I have a suspicion that it may only apply if the package is posted in the Republic.
Or maybe the postal services here haven't caught up with the Windsor protocol - though I doubt it.
I don't mind paying the charge too much - but I would have preferred it to go to Caroline!
Hope Dutch purchasers aren't similarly affected, but in theory the same regs apply throughout the EU, I think.