Ebay. And of course you are right about other things.
Over here in Blighty we have a website called "Parcel2Go" that finds the best price for parcels and it is amazing how cheap some of the deals are. Maybe US has similar. I've had good deals in the past off UPS and DHL - but not always. I've posted cheaply to Canada - but the customs people thought the guitar pickups were bombs and dismantled them. I had a guitar shipped by Houston Brass in the USA to me but that was expensive (and the guitar was total horsepoo that cost me £500 to get made playable). And there were some people in Miami who used to ship me Elixir strings for a total price including delivery that was less than the wholesale price in the UK - what could they have been up to (grin)?