Quantity filed is considered to be whole number, or an integer number and cannot be decimal.
If you need to allow this as a special case for your business model, modification is required.
Open file: core/lib/cart.php
Locate about line 305:
if ((int)$qty && ((int)$qty > 0)) {
if (!isset($this->session->data['cart'][$key])) {
$this->session->data['cart'][$key]['qty'] = (int)$qty;
} else {
$this->session->data['cart'][$key]['qty'] += (int)$qty;
}
//TODO Add validation for correct options for the product and add error return or more stable behaviour
$this->session->data['cart'][$key]['options'] = $options;
}
change to
if ((int)$qty && ((int)$qty > 0)) {
if (!isset($this->session->data['cart'][$key])) {
$this->session->data['cart'][$key]['qty'] = $qty;
} else {
$this->session->data['cart'][$key]['qty'] += $qty;
}
//TODO Add validation for correct options for the product and add error return or more stable behaviour
$this->session->data['cart'][$key]['options'] = $options;
}