Not sure whether it's the same in NI as in the rest of the UK, but
here's a cautionary tale, based on our experience in England:
If you order from overseas and customs decide to charge import duty, you
will be liable to pay that duty (fair enough) but also you have to pay a
fee (between £8 and £14) to ParcelForce or Royal Mail for them to pay
that duty on your behalf. ParcelForce won't deliver until you pay the
whole lot: duty and their own fee.
I've heard it suggested on discussion forums that you should get the
sender to mark the customs form "duty will be paid by recipient" which
means that customs will bill you directly (which takes a bit longer to
receive their letter and for them to receive your cheque) but it nicely
avoids ParcelForce being able to perpetrate their little scam on the
recipient. The fee is the subject of a LOT of angry discussion on
forums, as you can imagine!
Even better if you can get the sender to say that the item is
"commercial sample" or "gift" rather than "merchandise"