var submitted=false;
function addItem(id) {
        if (!submitted) {
        if (document.getElementById("quantity_"+id).selectedIndex==0) {
           alert("Please select quantity.");
           return;
        }
        if (document.getElementById("size_"+id).selectedIndex==0) {
           alert("Please select size.");
           return;
        }
        var txn = ""+Math.random();
        txn = txn.replace(".","");
        location.href = "cart.php?action=add&id="+id+"&quantity="+document.getElementById("quantity_"+id).value+"&size="+document.getElementById("size_"+id).value+"&txn="+txn;
        submitted=true;
        }

}