function update_total() {
  var product_quantity, subtotal, delivery, total = 0;
  
  product_quantity = document.getElementById("shop_orders_quantity").value;
  if (product_quantity == "") {
    return false;
  }
  
  subtotal = (product_quantity * 1.49);
  
  if (product_quantity < 2) {
    alert("Die Mindestbestellmenge beträgt 2 Artikel.");
    return false;
  }
  if (product_quantity < 20) {
    document.getElementById("mineralwasser-spray-display").style.display = "none";
    document.getElementById("shop_customers_mineralwasser_spray_display").checked = false;
    delivery = 5.50;
  } else {
    document.getElementById("mineralwasser-spray-display").style.display = "block";
    delivery = 0.00;
  }
  total = subtotal + delivery;
  
  document.getElementById("shop_customers_subtotal").value = document.getElementById("subtotal_first").value = format_to_currency(String(subtotal));
  document.getElementById("shop_customers_delivery").value = format_to_currency(String(delivery));
  document.getElementById("shop_customers_total").value = format_to_currency(String(total));
}

function format_to_currency(x) {
  var k = (Math.round(x * 100) / 100).toString();
  k += (k.indexOf('.') == -1)? '.00' : '00';
  return k.substring(0, k.indexOf('.') + 3);
}

function allow_only_numbers(evt) {
  var charCode = (evt.which) ? evt.which : event.keyCode
  if (charCode > 31 && (charCode < 48 || charCode > 57)) {
    return false;
  }
  return true;
}

// function allow_only_numbers() {
//   if (!document.getElementById("shop_orders_quantity").value.match(/^\d+$/)) {
//     // alert("Bitte geben Sie in diesem Feld die gewünschte Stückzahl ein.");
//     document.getElementById("shop_orders_quantity").value = "";
//  document.getElementById("shop_orders_quantity").focus();
//     return false;
//   }
//   return true;
// }