I'm launching a Suitelet from a SalesOrder record using a custom action as follows:

function launchSelectRateSuitelet(){
    var salesOrderId = nlapiGetFieldValue('id');
    var strUrl = nlapiResolveURL('SUITELET', 'customscript_id', 'customdeploy_id');
    window.location = strUrl + '&custparam_salesOrderId=' + salesOrderId;
}

as you can see, I'm adding a GET parameter to the suitelet, so it can access the record from where it's called.

Now, in the suite, i have a form that gets some additional input and submits it

function suitelet(request, response) {
    var id = request.getParameter('custparam_salesOrderId');
    var record = nlapiLoadRecord('salesorder', id);

    if(request.getMethod() == 'POST') {
        ....
        // calculate new values and save on 'record' Object.