Maybe they haven't the experience you require?
It all depends on the value put on the custom sticker. If it exceeds a certain value (I think it is £18) then you may be subject to import tax. If that is the case, you will also be charged £8 admin on top of the tax.
Sometimes, if you are lucky, you don't get taxed.
That is a general rule... not specifically CMoN (I have never ordered from CMoN).
The last time I got hit with charges, I spent $36 on miniatures (from RAFM) and ended up spending £14 on tax and admin charges (over two thirds of the cost of the miniatures).