Shipping a box to Japan?

I'm trying to ship a package from the U.S. to Japan but all of the usual shipping companies are charging way too much. They all say that their are express shippers and don't do sea or slower and cheaper shipping methods. The package doesn't have to be there tomorrow, anyone have any ideas? It's just a plain box. About 18X18X18 inches; maybe 20 pounds. Don't bother with DHL or FedEx.