As far as I can remember it works something like this. If you have something delivered from the US, it is liable for tax. The person who receives the item is the one who has to pay for it. That said, it is pure chance whether your parcel gets picked out by the the Customs or not. It is more likely that it would get pulled if it has a high value, i.e what has been put on the CN22 customs declaration form. If you are dealing with a private individual, you can get them to put a lower value, and also mark it as a gift. Of course, a business/company cannot do this, so again it would be more likely to be pulled.
I have had quite a few parcels sent from the US, and probably 90% have come through just fine. I did have one a couple of years back worth nearly $500, that I had to pay nearly ?75 excess tax on
, unfortunately there is nothing you can do about it, if you want your items, you have to pay the tax. Also, when I lived in Finland, I had a small parcel arrive from GHQ, I had to go the local port customs office, they wanted to charge 10-15 euros tax. I managed to get out of paying it, by saying I didn't understand what the customs official was going on about. 
Anyway, hope this helps. To sum up, it's basically a pure chance thing whether you get clobbered for the tax or not.