I am having problems exporting models to Shapeways. None of my weird exports worked, but then it turned out that the default sphere did not work either. Here is one that broke:

exports.potential = function(x,y,z) {
  x *= 1.2
  y *= x+y
  z *= x > 1 ? y/x : Math.sin(x)
  return 80 - (x * x + y * y + z * z)
}

exports.color = function(x, y, z) {
  var r = Math.cos(x*0.01)
  var g = (y+5)/10 * Math.cos(x) / Math.sin(z*4.4)
  var b = (Math.cos(z)+1) * z + 0.5  
  return [r,g,b]
}

I'm not especially familiar with STL so I'm not sure what could be going wrong. For what it's worth, I was unable to load the STL in this viewer I found, it ran into range errors getting data out of the model:
https://github.com/tonylukasavage/jsstl

Fun site though. It's neat seeing all your modules come together in a project like this. A GLSL/shadertoy type thing that you can actually print is so cool!