Different goods have different rates. Knowing that your shipment was worth $256 isn't enough to determine the amount you should be charged. Look for the US Harmonized Tariff Schedule. It's a nightmare to read but you should be able to find a couple chapters for clothing. I think one for woven versus knit. Depending upon what you ordered, and more importantly how the retailer declared it, 20% for clothing doesn't sound out of line.