No, that's right actually. That's all-inclusive- the tax is the customs charges, which may or may not apply (this page explains that customs uses a formula called "CIF" to decide the value for the purposes of tax calculation, and that something worth about $250NZD has a chance of having a high enough CIF to incur the charges). YouShop does not quote for the potential GST/import duty, but will still charge it. So, basically, not a rip off at all. Just remove the "taxes" component (which is charged by customs, not them) and compare that.
USD$250 is ~ NZ$305. You can bring up to NZ$400 worth of goods incl shipping costs without paying any GST. Duty is another component which may or may not apply depending on the item. I have never been charged GST for goods under NZ$400 incl shipping via shipito or via amazon directly. Borderlinx seems to add the tax component which you cannot remove. Yes the shipping is cheaper than youshop but if you cannot remove the tax component then it's more expensive. I understand that if you declare a 50" Plasma for USD$300, customs will tell you to get lost and charge you GST accordingly but borderlinx seems not care and add the tax based on the category you have selected.
They estimate what you may be charged and show you there - you would pay it to customs if applicable, not them. And again, read what is written about CIF. More than just the value of the item is a factor. Also duty, shipping, insurance, and any other charges. I just confirmed with their live chat that the figure you see as tax there is an estimate of what customs will charge you, and that they do not charge it to you.