function changeSum(num)
{
    obj = document.getElementById('sum');

    switch (num) {

        case 1:
            obj.value = " " + price_1 + " " + currencyText + " + " + shippingCost + " " + currencyText + " = " + (shippingCost + price_1) + " " + currencyText;
            document.getElementById('numItems_1').checked = true;
            break;

        case 2:
            obj.value = " " + price_2 + " " + currencyText + " + " + shippingCost + " " + currencyText + " = " + (shippingCost + price_2) + " " + currencyText;
            document.getElementById('numItems_2').checked = true;
            break;

        case 3:
            obj.value = " " + price_3 + " " + currencyText + " + " + shippingCost + " " + currencyText + " = " + (shippingCost + price_3) + " " + currencyText;
            document.getElementById('numItems_3').checked = true;
            break;

        case 4:
            obj.value = " " + price_4 + " " + currencyText + " + " + shippingCost + " " + currencyText + " = " + (shippingCost + price_4) + " " + currencyText;
            document.getElementById('numItems_4').checked = true;
            break;

        case 6:
            obj.value = " " + price_6 + " " + currencyText + " + " + shippingCost + " " + currencyText + " = " + (shippingCost + price_6) + " " + currencyText;
            document.getElementById('numItems_6').checked = true;
            break;
    }
}