Buying from the UK and shipping to the US, you should be able to find your answers here: http://www.customs.gov/
‘For instance, if you are returning from countries other than the Caribbean countries or U.S. insular possession you are entitled to an $800 duty-free exemption and the next $1,000 worth of the goods you purchased is subject to a flat rate of 3%. If the value exceeds $1,800, the remaining duty will be determined based on duty rates in the harmonized tariff schedule, which are generally between 0-10% (except for clothing and textiles, which can be much higher up to 25%) from countries that have normal trade relations status with the United States.’
I realize you are posting items, and not carrying them through customs but I would expect the duty tariffs to be the same.