The UK position is this:
* Plastic kits are not subject to import duty but they are subject to VAT. The current threshold for this is, I believe, £15.
* VAT is collected by Parcel Force, who charge £8 for this service.
* The Postal Services Act prevents them refusing to hand over your parcel until you've paid that fee. You can legitimately ask that they give you your goods and invoice you separately for the fee. However, the fee is legitimate, so if they do remember to invoice you, you must pay it.
Many people in the UK object to this fee. It's certainly unwelcome, and whether you've agreed to pay it is debatable, but this position overlooks a couple of crucial points:
1 VAT is a legal obligation and it has to be paid somehow.
2 Parcel Force's fee seems like money for jam, but it's a lot cheaper than arranging customs clearance yourself.
3 if we somehow, collectively, get it overturned, it will only be replaced by some other process that produces equal amounts of hassle and/or cost. This is inevitable because, well, see point 1.
You can sometimes ask for the value of your goods to be under-declared in an effort to avoid VAT. Some sellers even do this without your asking. WNW won't do it. I would strongly urge caution:
i) it's fraud.
ii) VAT is charged on both the goods and the postage cost, so you can't guarantee a win. (This is, incidentally, iniquitous. I have serious conceptual problems with VAT anyway, but to charge it on a service that was paid for overseas is beyond wrong. Still, it's what we're stuck with.)
Your better bet here is to hope that your parcel won't be inspected. Not all are, and I've heard that the dip sample is as low as 10%. The best way to avoid being sampled is not to order enough kits to fill a car boot - ask me how I know this!
As for the amount - VAT is 20% so £71 implies that your kit cost over £350. So I suspect:
I a misprint by Parcel Force, or the handwritten equivalent;
II an erroneous code, as Dave Cox suggests; or
III a misprint by WNW. I've had this happen with deliveries from other overseas sources - the label carelessly filled in meant that the value was horribly over-declared.