如何在Laravel 5中覆盖购物车总数

I am using gloudemans package to build a shopping cart. How do I override Cart::total() so it can output a different value if a user enters a coupon?

For example,

Cart::total() = 200;
$coupon = 20;
Cart::total() = 180;

if you use total method laravel shopping cart add tax to real total of prices just use Cart::subtotal() to get real to total of prices without adding tax to it.