function update_total() {
  var product_quantity, subtotal, delivery, total = 0;
  
  product_quantity = document.getElementById("ShopOrdersQuantity").value;
  if (product_quantity == "") {
    return false;
  }
  
  subtotal = (product_quantity * 1.99);
  
  if (product_quantity < 1) {
    alert("Die Mindestbestellmenge beträgt 1 Artikel.");
    return false;
  }
  
  if (product_quantity == 1) {
    document.getElementById("MineralwasserSprayDisplay").style.display = "none";
    document.getElementById("ShopCustomersMineralwasserSprayDisplay").checked = false;
    delivery = 2.00;
  } else if (product_quantity == 2) {
    document.getElementById("MineralwasserSprayDisplay").style.display = "none";
    document.getElementById("ShopCustomersMineralwasserSprayDisplay").checked = false;
    delivery = 2.01;
  } else if (product_quantity < 20) {
    document.getElementById("MineralwasserSprayDisplay").style.display = "none";
    document.getElementById("ShopCustomersMineralwasserSprayDisplay").checked = false;
    delivery = 5.70;
  } else {
    document.getElementById("MineralwasserSprayDisplay").style.display = "block";
    delivery = 0.00;
  }
  total = subtotal + delivery;
  
  document.getElementById("ShopCustomersSubtotal").value = document.getElementById("SubtotalFirst").value = format_to_currency(String(subtotal));
  document.getElementById("ShopCustomersDelivery").value = format_to_currency(String(delivery));
  document.getElementById("ShopCustomersTotal").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 : evt.keyCode;
  if (charCode > 31 && (charCode < 48 || charCode > 57)) {
    return false;
  }
  return true;
}