How to set the price level as a custom only for a particular shipping method

When the shipping method value is “Sample” the line level price level is set to custom. By default the rate should be set to the value 0.00.

function postSourcing(scriptContext) {

    try {

        var currentRec = scriptContext.currentRecord;
        var shippingMethod = currentRec.getValue({
            fieldId: 'custbody3'
        });
        var sublistName = scriptContext.sublistId;
        var sublistFieldName = scriptContext.fieldId;
        if (sublistName === 'item' && sublistFieldName === 'item') {

            if (shippingMethod === 12 || shippingMethod === '12') {
                var price = currentRec.setCurrentSublistValue({
                    sublistId: sublistName,
                    fieldId: 'price',
                    value: -1,
                });
                console.log("..price..", price);
                currentRec.setCurrentSublistValue({
                    sublistId: 'item',
                    fieldId: 'rate',
                    value: '0.0'
                });



            }

        }


    } catch (err) {
        console.log("....error@PostSourcing", err)
    }

}