public function getDeliveryCost()
{
$cost = $this->delivery_price;
if ($this->isCouponsAvailable()) {
$validCoupons = $this->getValidCoupons($this->getCouponsCodes());
foreach ($validCoupons as $coupon) {
if ($coupon->free_shipping) {
$cost = 0;
}
}
}
return $cost;
}