I need to implement a way to show/hide shipping methods based on certain criteria, such as post code.

I have no idea where to start with implementing this. I mean, the scripting logic shouldn't be too difficult, but I have no idea how scriptable cart works. I can't even get a simple pop-up to trigger!!!

This works on the Sales Order I created when loaded within netsuite, but doesn't do a thing on the website

// JavaScript Document
function sale_init()
{
var pcode = (nlapiGetFieldValue('shipzip'));
alert(pcode);

}

I'm calling it on page init and field change, just to see if I can get it to trigger somehow on the web, but no joy.