OK So I have been using the following threads to help create a script / custom fields to calculate total order weight for sales and invoices.

Client side script for total order weight?:
https://usergroup.netsuite.com/users/showthread.php?t=4695&page=3

Item/Quantity Total on Packing Slip:
https://usergroup.netsuite.com/users/showthread.php?t=4888&page=2

I have created the following scripts:
   
"calcTotalWeight" called in Save Record Function and Recalc Function

function calcTotalWeight()  {  // Called in: RECALC   // On Recalc (after Validate Line returns True and the line has been added/inserted), loop through  // the line items, adding the line item weights to the total weight, then display the total in   // the Order Weight field (custom transaction body field).    /* -------------------------------------------------------------------------------------------------*/       var totalwt = 0;       for (var i=1;i<=nlapiGetLineItemCount('item');i++)     {        var line_weight = parseFloat(nlapiGetLineItemValue('item', 'custcol_itemqtywt', i));        totalwt += line_weight;     }       nlapiSetFieldValue('custbody_orderweight', nlapiFormatCurrency(totalwt));    return true;  }    /* -------------------------------------------------------------------------------------------------*/