I shipped my Roland digital piano by surface along with a bunch of
other stuff.  Is the piano the only thing you're shipping?  If you've
got other stuff to ship and via surface they basically ship by volume
not weight, so a dense item like a digital piano has good cost
performance.  If I recall corerctly it cost me around $800US to ship 2
cubic meters door to door (to Japan).

Otherwise, as someone elase mentioned maybe you could check it in on
the plane and spend that money instead on a decent road case - though
you've probably have your hands full already :-)