US Customs isn't worried about $2 LEDs. They care about $2,000 art pieces.
You can read a bit about it here but it's not an issue with USPS, it's with US Customs. One segment of the page states in regards to exception from duties: "On mail items examined and passed free of duty by U.S. Customs."
712 Customs Clearance and Delivery Fee
As a seller on ebay, I say the same thing to non-US buyers. I only ever had one person complain about owing customs duties over in Europe someplace. Wish I could remember what it was. It was their responsibility to pay to import it, not mine. And anyway, you can I think simply reject to pay duties on something, but you don't get to have the item. It gets sent back to customs for destruction or something like that.
I have seen a TV program on Australia's customs service at an airport. I assume the US operates in much the same fashion.