According to customs and excise (http://www.hmce.gov.uk) you are only allowed to be sent £18 of goods before you become liable for duty. This is if you order something from outside the EU e.g from the Internet in the US or if you get something sent as a gift from the US.

You could get your relative in the US to ship it to you marked as a gift with a zero value (or under £18 value) on the customs label. But if customs in the UK open it up and inspect it you could end up in trouble. Also if the post service lose the package you will not be insured since you marked it up at a lower value.

I used to order quite a lot of stuff from the internet in the US and at one point I was getting around 1 in 5 packages opened and inspected by customs so it could be risky doing this. Your other option would be to get somebody to fetch you some back from the US in their hand luggage (although this is also risky since you are only allowed £145 of goods).