Rio is absolutely right. I brought in a guitar worth >70k for no duty, but a bunch of capacitors got charged at 22%, or about 800 bucks for a 100-dollar order. The high-end 200 dollar amp kits and chips went scot-free, as did a recent order from DealExtreme. But when I got speakers imported I again got charged 35% or something.
They have monthly targets I think, and they figure out ways of meeting them. It is totally your luck as to when the shipment comes in.
You can contest duties and ask for reimbursement, but I'm not sure if the procedure is totally simple and transparent, and therefore worth the hassle.