Check with up-to-date local laws, it will become clear what you need to do. Laws do change in time, and you cannot rely all the time on your past experiences.
"Unpaid costs" are most likely duties and VAT on the model. You import from outside EU, so you need to pay those to your governement. What you already paid was only item cost and shipping, since shops do not prepay duties.
Nothing of that is HK SpeedPost fault, from your description it looks like you failed to understand import process for your country.
As for buying abroad and importing, I had good experience over ten years, various countries (including UK) and continents, various package carriers (including many times with HK SpeedPost), various sellers and shops (including Redwolf), changing laws (pre- and post-VCRA). It was enough that I knew current law and made sure to follow correct process. Even when packages were held by Customs it was easy to retrieve them after providing clarifications, or paying duties/VAT.
The only time I lost a modelgun (and a rare one) in shipment was when a seller failed to follow simple shipping directions, and additionally failed to stand up to his words and reimburse me when model was confiscated due to that.
He conveniently blamed everything on Post, as far as I remember. Talk about contradiction in words...